Ah, that makes a lot of sense,
Lots of the game's design choices only make sense in the context of a 5v5 team battle, but I think it could be pretty cool.
Also sorry if my wording was too harsh, I was just really passionate, since truly want to see this project grow into something cool.
Though I would like to reiterate that one of my biggest problems with the game was that it just takes too long to complete a match, even on easy mode. I really suggest either adding a speed up / skip turns feature, or scaling down the HP and damage to smaller numbers to raise strategy and risk - while also making the gameplay less slow.
Since you mentioned one of your goals was to have synergizing characters, I have a few ideas that might give you some inspiration: taunting, damage redirection, thorns effect, giving turns to another character, gaining ability stacks from being healed or taking damage, enemy immobilization (voring opponents?), and having more AoE damage.
Those are just a few ideas I came up with quickly, you can think about them if you want. I would suggest looking at other turn based RPGs though if you want another look for inspiration or game design. I know it may sound silly, but my "toughness bar" suggestion actually comes from Honkai: Star Rail. Different attacks do different amounts of toughness damage, and when the toughness bar is broken, the enemy takes more damage for a while. An important part of game design is looking at other people's projects in order to understand what works and what feels good. There's lots of RPGs to pull ideas from.
Take everything I said with a grain of salt though, I don't know your project or vision as well as you do, but I can playtest and give suggestions. But at the end of the day, the thing that matters most when making a game is actually getting it finished, and getting it playable. And more importantly, a good time. Don't get too caught up in adding too many things at once, since too much overcomplexity can easily lead to a project falling apart. Just do whatever you feel like, and have fun making it. That's what matters most.
I'll be eagerly waiting to see what you do with your game moving forwards.