Looks like most of what I'd suggest has already been said...and answered, but I'll repeat anyway:
- Visual and audio feedback as to when enemies are about to move (some people don't see as well, some don't hear as well, or play with sound off).
- Separate keys (or buttons) for each action and for pause would make it (I think) a lot easier to learn. Most platformers require you to learn a bunch of buttons: not so many that require you to use a menu in the midst of gameplay. And you have several levels that require you to jump twice in a row, but if you want to do that immediately from a pause it seems like you have to cycle all the way around?
- I didn't notice anyone else saying this, but I'd like 'R' to restart the level directly, rather than kill you and then you have to press it again to restart.
- It's a little weird that you can kind of use the jump ability multiple times, but I don't see that there's anything you can do about it: it enables a lot of good level design.
- Checkpoints might be nice, especially in that last level. Different people have different tolerances for how long they're willing to go without a checkpoint and some games demand that you go a ridiculously long time between checkpoints (especially boss fights: Hollow Knight and Cuphead, I'm looking at you). But this one pushes the limits of what I'm comfortable with and you could make it more fun for more people by allowing them to take it one piece at a time. And you can always turn some off in a higher difficulty mode or something.
But yeah, those are polish things: for a 48hr game this is excellent. Jam games are almost always too hard and a little unpolished.