Hi. I came here from your last YouTube video. Undertale is one of my favorite games of all time, but I feel like the formula is not as fresh as it used to be. The keyboard/controller controls seemed boring and brought nothing new to the table. Now, the mouse control caught my attention, so I stayed watching and finally decided to give it a shot. After all, it was what made me click on the video in the first place.
I really enjoyed the experience. I feel like this really is a unique experience, different from other RPGs that use bullet hell mechanics, and shoot 'em ups, in a good way.
Starting the battle is satisfying and the control is really fun. I also agree that the more deterministic and phases-based combat is more rewarding. When you die, you don't start from scratch, but with the knowledge you've learned from the previous attempts, which feels really good. And the fact that it's not 100% deterministic keeps you on your toes, making future rounds not trivial and still engaging.
Now, I think a small grace could be given after the player takes a hit so they don't take multiple hits in a fraction of a second. I must add that I'm not that good at games, and although I beat the fight, it was hard and sometimes it felt unfair. With that said, I think that if what you mentioned in the video ends up being true, that as your stats get higher, the previous encounters become easier, then that could work for me, similar to how other more traditional RPGs work. Maybe then, getting i-frames after taking a hit could make the game too easy. Something to think about.
Finally, performance was flawless, but I also run a Ryzen 5, RTX 4060 and 16 GB of RAM, so take that with a grain of salt.
Good luck with development!