try to avoid layering the black outlines of objects when they connect and you want them to bounce, it throws off the flow of it and makes it look weird
i would also say make the black outlines a tad smaller
and yes this would go good with some SFX, you could try searching google for ''royalty free sfx'' or something like that to get some, you can use royalty free samples in anything so those are great to have with animation programs
keep it up my dude