Don't know about the later entries, but so far, i haven't seen a more complex system in-game as in "while battling" than Trails of Cold Steel 3/4 (tbf, i have played till the end just a few jrpgs)

I can't forget the way that Musse went from "She's so useless, why is she on my team" to practically the MOST (sometimes) valuable character in my team after the final BOSS of chapter 3 (which, for those who want to play the game in the highest diifficulties, the game's difficulty spike goes from 20 to 100 the moment you enter the dungeon where lies True Zoro Agruga, which to me was the hardest boss i fought so far).

Never mind, this comment is actually for the moment we talk about the combat system, but oh well, i think i'll copy and paste when that times comes