Hm I see, Jean was definitely always on the edge of OT (well, generally not, I admit it happened after Isaac died). OT sharing would be helpful. And the cap is high to allow other skills to use it. But I admit, considering OT charge speed, max OT requirement would be 10-15. Unless OT charges faster with level? Just be careful with scaling over factors of 10 between skills: it will make the first skills work with super cheap OT, and player won’t be able to compare skill values reasonably in advanced game, possibly leading to useless skills (in RPG it mostly happens with items, such as Potion healing 50HP in end game where you have 9999HP) or OP skills. But I’d need to see the whole system and skills to be sure. You can have a non-spoiler playtesting with people by having them play advanced battles without story context (the difficulty is that they need to learn all the systems and skills to be able to test properly).
The difficulty spikes were the first mini-boss where you learned Rage (maybe it’s a good thing? you have to Defend properly both normal attack and Rage to go on) and the boss after that (although that’s to be expected).
My biggest issue was the battles getting longer and longer: if Jean dies, you will die soon unless boss has little HP left, but if Isaac dies, you can survive by using Jean to heal herself, Defend, repeat in a survival way, and doing little damage. So it’s not very interesting and I gave up and allow my character to die. This is because there is no way to revive an ally, which is also true in early game of many JRPG but those have items to revive instead: this gives you a few chances to revive other characters, and once you consumed all revive items, this time you generally die pretty fast even with a healer left.
Even if you did things properly, battles were quite lengthy. After 3 loops I get the system, it’s all about repeating the same things without losing my concentration. Since the enemy HP is not displayed I don’t know how much it will last and so the battle gets more tiring until I accidentally do some mistake and lose a character. So I’d rather have short battles that ensure you win if you understood the enemy patterns correctly and did the right thing 3-4 times. Of course, with a deeper system, more characters and skills, and more boss phases you can justify a longer battle, so maybe later in game.
I do see in the Tactical Guide that there are boss phases, but to be honest I didn’t grab the subtleties of them. Real-time battle makes it harder to pause and think of course, but I do remember that even Chrono Trigger had very clear boss phases. This may require more budget with dedicated boss sprites, but I think it’s also design and UI/text indications that made them recognizable (counter-attack was very clear although not super interesting, Juggler switching between physical and magical stance was done via text but graphics could help too, Magus switching elemental barrier via an actual skill use + text).
If there are only meant to make the boss a little stronger and not meant for the player to react in a particular way, then I suppose subtle “mini-phase” changes are fine, though.