Good job on getting so far without upgrades! The game is designed to be very punishing, almost impossible to beat without upgrades since getting stomped is part of the fun. So you're right in that it is kind of unfair. I will slow down Fabienne's grinding stomps for the next update though.
idk if tuned down is the best idea. Maybe this attack their hitbox is smaller because she only stand on her hoof, an slight tilt to notify, "hey this attack would hurt" (you can use the one already present). I always find it weird that even if she stand up on her hoofs. Her hitbox is as big as the bunny. Being able to evade easily in any direction would be cool. Or atleast it would knock you down instead of losing life. Like it hitted your head but not your entire body.
You can still evade her of course going up but it just feels weird for her design. I can't calculate well when did I evade her succesfully
Another way is to modify the sound cues of when she is gonna make a grind attack. It would feel less of an cheap attack. And it would make the player panic a little using more stamina trying to evade her attack.