I've never played either Gunvolt or Celeste so the comparisons are unfortunately going to be lost on me.
I definitely feel you about getting used to things during development and needing players to test them to get a useful opinion on them. I've had actual bugs that I just kinda got used to and forgot about until people told me about them.
I tried the slide a bit but I couldn't really find any use for it. That makes sense that it would be used later when enemies are in the game. I haven't played the games you mentioned but I think I understand what you mean here, it sounds like some games I've played or seen played.
I definitely did get the impression that it was meant to be more about speed and maybe rhythm than precision. More of a Sonic than a Mario I guess. I think as it stands it's too easy to slam to a halt but I think you already mentioned that. I think it could be a lot of fun though obviously there's still a lot of work to do to make that happen.
Playing with sound definitely helped- the first time I played I thought I was going mad because I didn't realize the level was supposed to be changing. It still really needs an indication of what causes the change (am I hitting something?) and what the change is.
The level lacks landmarks which makes it hard to tell where you are, but I'm assuming that's mostly because it's been thrown together quickly. I think the biggest thing I don't like is the layout- it doesn't really seem to have any direction to it. I feel like I'm supposed to be going up if I'm climbing a mountain but it's full of weird loops and backtracking.
I'll try to give this another shot if I have time.
Do you have any plans to develop this further?