With no way to interrupt spell casting I don't think I want enemies to cast the same spells the player can... because then the "win" button for the AI is teleporting your 2 main characters into lava. To quote Aliens "Game over man.. game over"
BG2 tactics was also not balanced because enemies had always max spells, but you only had that if you rested after each engagement.
Either way, better AI sure.. but the issues are the game systems, not so much balance or AI. The magic while nice to look at is not structured properly, there aren't counter-spells, spell mantles, spell triggers, higher summons, delayed action spells etc. And to boot there is no way to interrupt spell casting. Meaning that teleport spell for an enemy caster? Would always 100% be game-over.
Somewhere you have to draw a line in the sand with spells that the AI absolutely should not have. In tactics, I drew that line several times with summonings. While lore wise correct, higher undead or demonic summons can absolutely and fundamentally change the dynamic of a fight. And tactics is really not fun at all once you reach level 18+ and encounter enemies with lvl 8 or 9 spells..........
I think if anything, combat needs to be balanced not by numbers, but by options. The AI should have a counter to everything the player has, but the AI should not have things that do not have a counter (like teleport or feather fall)
Remember the end-goal is to make a fun game, not to make a game only 5 people will ever complete. (Which is definitely true for the bg2 tactics mod ,p , assuming you played on the highest game difficulty)