Great devlog on YT and a really elegant concept. I think, you should really go for it and make the finished game!
Played the demo on PC, beat it the first try, but then lost 3 times in a row. Overall feels quite juicy and satisfying to beat. I guess the first win was due to having already seen the pattern in a video :)
Some thoughts and suggestions:
1. Maybe give the player a bit of a breathing room between enemy phases. That could help with the feeling of progression (for every battle/enemy type) and pattern memorization, but also having a constant stream of attacks doesn't seem quite fair
2. As people have already suggested, having anticipation indicators may also help with combat feeling more fair. But honestly, players can appreciate the harder "die and learn from your mistakes" type of combat as well
3. Idk if this will be relevant, but having different mouse-sensitivity settings may result in players having different experience with the game. Idk how to approach this problem, but you could lean into it and design an encounter specifically for higher/lower sensitivity and run a demo again :)
All in all, this is a very inspiring concept. Worth investing your time into. Cheers!