Thanks for the detailed response! OT charge remains the same, though skills increase in power every 10 levels. The majority of skills cost under 10 OT, with those exceeding it being really powerful skills that you are encouraged to be using the distribution/manipulation skills for in order to use.
I'll have to experiment with giving Isaac a Revive skill - maybe changing the 'Survival Skills' skill to have an alternate effect if the ally is KO'd since Jean doesn't get her Revive skill until 10 - though that doesn't solve situations where he's KO'd. Maybe Jean's Restore can revive an ally if they're KO'd as an additional effect. Perhaps I can change Jean's Bio Seeker effect to make an enemy's HP permanently visible if the OT gate is met (with it not affecting damage otherwise), rather than have it only display in a message after using the skill.
There's definitely a lot of things I haven't considered as I'm sure I play the game "optimally" and so don't really know how others play or struggle with it aside from 1-2 playthroughs with the modern system I've seen. I'll have to look into setting up some kind of non-spoiler battle playtest build. Right now my priority is just putting everything together and then balancing it.
Regarding battle length, I'm concern that if battles are too short, I defeat the purpose of having them, since each encounter is fixed rather than something that occurs randomly multiple times in the field. The game's biggest inspirations are Yggdra Union and Etrian Odyssey, which are both pretty challenging, though because they're turned-based it's easier to take a breather and assess the situation, rather than using the 'Pause' overlay, which may feel like an inorganic method of accomplishing that.
The "phases" are mostly weakness shuffling/Guard Point changes. There are some "phases" like bosses gaining additional actions or changing their timer after being Dizzy'd a certain number of times. Maybe I can modify part of the UI to make these changes more visible. Since the game is so fast, sometimes those messages aren't retained because the player is busy choosing actions.