The change I'd make here is to make it so someone in the party engaging a boss doesn't immediately put YOU in combat and take you out of Hide.

I understand there might be a balance reason why opening a boss with trick attack would be bad, but it really feels awful to just suddenly not be hidden anymore because someone else poked the boss.

The cooldown on SA and TA is fine and keeps Suiton from automatically being the correct ninjutsu to use when you don't have to use Huton. Nothing wrong there. Just the initial opening on a boss, being pulled out of Hide feels bad.