I really dig this concept and the execution is pretty satisfying once I get the hang of it.
Initially I was overwhelmed with having a full party and list of spells, I wasn't sure how to target specific heroes for a while, it's much easier to click on them in the menu since there isn't as much feedback clicking on them in the overworld (e.g. a highlighted aura for heroes I mouse over)
I assume in the ideal game you'll have a smaller party at the start and less spells.
I think the mana regeneration could use some work. It makes sense as a way to punish a poor rotation of spells but since I can get it back by just waiting around I don't feel pressured to go into the next fight unprepared so I'll just wait around. It might as well just refill automatically after every encounter. Maybe if I had to siphon enemy MP or search for potions instead? Something more active could be interesting.
A feature I would like is the ability to change targets mid spell channeling because there were often times where I'd want to swap my cast to a different party member as the battle changed but was unable to without canceling the spell, even though I was allowed to click on them it would only affect the person I targeted when I started.