I tried to change the controls so that K is dash (since that's what I've got it set to on Hollow Knight) and moving uses WASD, but I can't dash at all while holding W and D. I can dash in every other direciton using WASD, but not up and to the right at the same time. I can dash up and right using the arrow keys, but obvs I can't switch to arrow keys every time I want to dash that way and I suck at using arrow keys as movement controls. help?

so I ended up changing the dash button from K to right shift to try to use the arrow keys only when I needed to dash up and right (which is a really awful setup but I was desperate to play this awesome game), but as it turns out all the directions work just fine with WASD when dash is right shift? I did try downloading the openGL version to see if that would help (it didn't) so I have no idea if this works on the other Windows version, but if you're having this problem try setting dash to a different button and see if it works I guess

This is likely a problem to do with your keyboard and not a problem to do with Celeste, actually. A lot of keyboards don't support a lot of keys held down at the same time.  I'm glad you managed to find a workaround by setting the buttons to different keys! That's pretty much all you can do unless you want to invest in an expensive n-key rollover keyboard that doesn't have such problems.