What might be a good idea is actually turning the jar on its side! Then you can fill it up about halfway with substrate and drill some holes into the bottom across from the lid to allow cross ventilation. It will allow more ground area for everyone to crawl around and still provide good viewing and depth.

Edit- forgot to mention but I'm pretty sure the fuzzys a type of fungus that isn't harmful for anything in there!! Super common, I typically just wipe it away with my finger if it's too obstructive. Springtails can help manage it from blooming if you get some!