Hmm, as the game is right now you should be able to do a double jump while holding left or right, the platforming is predicated on you being able to do that. However, perhaps you're walking off of the platform as you do your initial jump (so it thinks youre doing a double jump), which is actually something I want to fix so that you have a few frames of leeway (games like Celeste do this). You're totally right about the sliding, I can make that a bit tighter for sure.
Thank you for the feedback! I'm glad that you think the game is great and we'd love to make it even better. I'll try to incorporate your feedback (: