Better well rounded team

Did you mean rerune because you don't think she's good as you have her, or the mon itself? Because she's either the best or second best healer in the game, only Ariel is really competition for that. She is a nat 5 in a nat 4's body. Yours as you have her, no, because she needs to be triple HP, I've seen spd/hp/hp for people with elite runesets but they're well over 30k hp with the spd rune while maintaining Violent. Never, ever give her a defense rune, and you shouldn't be using a flat defense (or hp, or atk) rune for any reason on anyone. Doesn't matter it's a 6 star, and would not matter if it started out legendary, they're just very, very weak compared to % runes. I've made the example before of using a flat 6 star hp rune on a very low base hp mon, like the wind elemental, and % hp is still miles ahead. On someone with good base hp, it can be one third the strength. This also has nothing to do with guild battles, she needs those runes regardless of where she is, though I tend to see the spd/hp/hp suggested mostly with PVP in mind, her PVE speed can be perfectly fine just grabbing spd subs usually.

The only acceptable reason to use one is when a 6 star is a legendary with spd substat that rolls 3+ times and you intend on using it with a mon that cares about speed racing and nothing else, which is a non-universal niche for Bernard and, to a lesser extent, Chloe, depending on where they're used. In some cases they try to go first and that's the win or loss, some people have 2 Chloes or 2 Bernards specifically for this reason, so they can have a crit dmg slot 4 and flat atk slot 6 on Chloe and don't care because they were both 20+ spd substats. I don't think it's really a good idea when only maintaining 1 copy of the mon.

Arnold's okay until making the despair/violent choice, Bella's fine until Violent/revenge or /nemesis, though he's going to feel squishy in PVP with less than 20k and still is missing 25% acc (or more, there are rumblings on reddit from people unconvinced they haven't changed res/acc calcs and 85 isn't a mechanical cap anymore). Vero will gain an enormous amount on Violent when available, I favor the spd/hp/hp with acc substats gearing route myself, that's his most consistent performance-wise. As his skills go up, the acc substats become more and more important.

Notes on your current 5s:

I don't like Dagora since he's so slow at soloing and doesn't contribute elsewhere besides as a giant wall, but as an FYI, he should be triple HP or possibly a single defense rune. Not sure on the math for his longevity there. Atk is pointless, his dmg scales off hp anyway so you're getting 2 things with 1 stat. His heal is also based on his hp, as is his rez. Not sure I agree with Despair on Tesarion, but I think a tank one instead of yolo is correct. He's only an average nuker, not terrible but not amazing, making him tanky instead lets him be used more safely to neuter mean passives in PVP. Very little use outside of PVP because they tend to disallow oblivion on anything in PVE worth oblivioning. Jean's fine except for that slot 6. +15ing a flat defense rune was a huge waste of mana. Needs more crit substats, it'd be nice to get him in the 60s to a flat 70 so he was autocritting on skill 2 without a buffer. If only ever using him with a Sieq/Aquila buff, then 40's actually pretty ideal, but if you were buffing via a Megan or something, it would feel too low to me, missing with a 72% surprise visit sucks.

Shannon's the right idea, just needs more #s of everything. Silia I'm not a fan of, no real advice for her. Jealousy is so fussy, especially in PVP where a lot of times 'beneficial effects' is immunity, negating the shift from 50 to 100% chance. I haven't seen much out of Velfinodon, but all the lizards are better than average nat 3s, def scalers do very well when they're avoiding ignore damage. Worse to worse they always excel when using vampire runes, scale so well with them. Bernard needs a lot more spd subs, his slot 4 isn't that huge a deal though. He scales well with def and hp both, the giant hurts when he's missing his atk debuff, so 53% accuracy will not feel fun compared to getting it capped. Since you're already using an acc rune, just finding a 6 star one would finish that problem.

Darion is triple def, he needs to be triple hp or hp/cdmg/hp. From what I understand, the math on him has hp/hp/hp while focusing on both crit and cdmg subs to be superior to hp/cdmg/hp even if you want him to be more dangerous, but I think that's at the very high end where 25k to 30k on him is just so squishy he'll disintegrate. He scales damage off hp for his first skill, and is mostly used in PVP, there is no reason to ever have a defense rune on him, he's not like Dagora where you might consider two hp and 1 defense for just pure self-sustain (and I'd probably still just go full hp there). The Violent part is correct, though. Ideal should be violent/revenge. Triple revenge is too fussy, I think he's much scarier when he can have more RNG on his side when being ignored, otherwise it's the fire monkey king problem.

Ahman's fine, though he always needs more of everything. Gets much improved when able to transition to Violent, but he'll still pale to Chasun overall, he's more of a situational "how much will provoke or his damaging swings help here", though I do sometimes pull him out despite Chasun/Bella. Sometimes complementary, or with a very slow and controlled team. I've used vero/bella/ahman where I let the combination of violent procs, def break and provoke whittle a GVG team down. He can be a huge counter to DPS with dangerous 2/3s, like Zaiross, and any healer. Also helps he can heal while locked by Zaiross or Delphoi or anyone else.

Chasun I mentioned already, and should definitely be 6'd since she goes literally everywhere (I use her the most of any mon, she is in basically 100% of my arenas, always in 1 of the 2 fights in GW, GB10, every ToA floor, ToA hard *if* I bring a healer, DB10 despite being wind, every HoH, etc). The only thing she can't do is farm. I would highly suggest triple hp, but if you have a junk slot 2 hp rune, spd/hp/hp will be okay, especially if you're using energy over swift. Spd/hp/hp is probably ideal if you're triple energy. If you go swift (she wants violent desperately, but until you have that), then I'd probably stick to triple hp. She is an amazing fire tank, helps her do that. Whenever teams have Pernas, etc., Chasun's tanking and self healing. More dangerous than using an Ermeda with a pocket healer, but super efficient with 2 mons handling that 1 thing. Technically if all damage is going wind, Ramagos accomplishes the same thing, especially in guild wars, and is usually able to do the brunt of the killing too, but that's all he can really do besides solo scenarios slowly.

You have very few 6 star runes, and I'm not sure if I remember seeing any 2/4/6s that were 6 that weren't junk flat stats. You don't need to worry about DB10 yet. Violent runes are great, but you will need giant runes for some mons' best-in-slot sets, Despair for a lot of ToA and CC based mons, Focus and Blade for a ton of mons as their 2 pieces (and some will never want to swap to an effect based one like nemesis or revenge, or can go either way), swift for speed racers like Bernard/Megan/Chloe/Verdehile), rage for burst-oriented DPS like Jean/Lushen/Kahli, and so on. Having 6 star runes will be an enormous power upgrade for you. Additionally, it will be a while before you can do higher dragons. Veromos will carry hard, but he needs violent and faster than he already is while on swift to properly carry. Once you have GB10 on lockdown, you can likely take your GB10 team, assuming it has Vero, and just... do DB8 with it effortlessly (thanks, water element), though you have to deal with an invincibility tower, and B9 may even be easier in terms of setting up auto because B9 would allow you dip into fire (it's dark, can bring anything) and B9's right tower is an ATB booster, it's easier to just rush the dragon down. No fiddling with the right tower like DB8/10 without dispellers like Bella/Megan, and autos usually need 2 of them together to properly dispel if rushing dragon.

As for who to work on next, Chasun and a GB10-capable DPS for way faster rune times. Your only real option is Julien from what you have.

Veromos/Chasun/Shannon/Bernard/Julien would be significantly faster than Veromos/Chasun/Shannon/Bernard/Bella. Technically Veromos/Bella/Shannon/Bernard/Julien (or whatever wind DPS) would work, but it requires stricter runes than solo Chasun. Keep in mind it isn't as safe, single healer and no sieze anymore, but it is much faster, and would be possible manual when the slower route was on auto (or possibly before). My only problem with the Julien suggestion is he changes so, so much with skillups, and I don't think he's worth devilmonning, especially while holding other mons that need it (finishing Vero, finishing Chasun's 2/3, she gets WAY stronger with a cd removed from both heals). He's like 3 months away, at least, assuming average skill-up luck on Chasun, and I still don't like the sound of invested devilmons. The only other option might be a slow-and-steady (but still faster than double healer) with Veromos/Darion/Shannon/Bernard and Chasun or Bella. Darion would buff you in PVP when 6'd, but doesn't have that many PVE applications. He's sometimes a viable candidate in HoHs where you need slower, sturdier DPS and supports to survive on B2/B3 autos, especially if Bella isn't in that team and you need a def-breaker leading focus fire.

For non-GB10 strength, it is without blinking Chloe, Baretta and Hwa, and having a rez option is nice, so that flags Briand, and even with Vero, occasionally Konamiya finds use places. Chloe can potentially be in DB10, depending on comp, but regardless, she is considered one of the top 5 or 10 best mons in the game, probably the strongest nat 4 period. Baretta is a ToA god, and also can be part of DB10s depending on whether you're going straight boss or towers and if you have Verde, and Hwa also has a place in DB10s, particularly in god comps combining Hwa with Verde if you ever pull him. Major strength in those fire mons. I would focus on GB10 first, but you have real muscle there.

Elucia has no real use besides that whole starting-monster-trophy, Susano is a potential single DPS in DB10 and a good vamp farmer but nowhere close to a priority, Lulu actually always has potential purpose so I'd never destroy him but not at all worth leveling right now, Julie is... one of the most disappointing 4s in the game once you have a lot of other 4s, Megan is potential DB10 material and a great arena wing farm assist, Rina is useless unless you need a wind tanker for AO or GWO, I actually love all lich kits (personal preference, such neat designs) but he isn't worth building atm, all lizards are solid but I wouldn't work on Igmanodon atm, fire Pierret is disappointing, Fria even with her buff is disappointing, Arang can actually hit pretty hard but I think she's better used as fusion, Michelle is getting an AI buff but I'm not sure how much use she'd see when you have a Briand too, harpy is meh, Gin is very meh (thanks, voters), fire bomber is abysmal, and, since I see it hiding, some people have actually done really interesting things with Miho for PVP, but that's for down the road. That's everything I could see. Only mon in here I probably should've listed with the ones above is Megan, but that depends on how the DB10 team is built.