Hi there! I honestly don't remember much about the frame rate being too bad. They did release a PS5 update to fix some things. I do remember the last boss fight being completely out of focus. It's been almost a year since I played this. Sorry I couldn't be much more help.
Yea this game was definitely much better than I anticipated, it's one of the better N64 RPGs for sure. The Ending was a little lackluster I agree. It does have 2 gameboy color games though!
It is not a 100% playthrough. I just take a really long time to play these games haha. I will be getting to them at some point, but I have a huge backlog so it might be a few months before I get CS4.