Thank you Zay for the feedback! It's incredibly valued.
There is an invincibility window but it seems that I put it too low (0.2 seconds) in order to be properly noticed. You're absolutely right on that because it can feel unfair to receive a lot of damage in a short amount of time.
I'm also glad that the progression and adaptive difficulty felt good for you! The difficulty is actually one of the most controverse topics I had among the people that playtested it.