This took a lot longer than it should have because there is apparently a bug where the player won't always hit the platform due to gravity, so I had to implement grace frames simply for the collisions between player and ground to work as expected...

(this was not my previous experience writing a platformer in Godot; I'm guessing it's because I'm using very small pixel numbers for everything)

I should probably go to bed now; hopefully progress will go a lot smoother tomorrow.