I think it can be as simple as simply mapping it to the Shift button - as pressing jump should always give some upward velocity. In the game I'm usually moving towards the right/left , so clicking Space (or any jump button) in succession gives very different movement feedback, which is unintuitive. Celeste also greatly rewards the horizontal dash -> wall grab combo, while your game seems to favour upwards/diagonally upwards dashes (which are basically double jumps), with horizontal dashes resulting in falling into pits - so in this specific game you could ditch the dash mechanic entirely and have a strong double jump with just the jump button, and keep the shift for speeding up horizontally.