Those rocks are snazzy! In terms of suggestions, I'm an advocate for 'cosy' spaces for axies, but it could just be mine that love to crawl in and around small dark spaces! A couple of mine like to swim, a lot, so I've left the front of the tank completely free of 'stuff' so they can come and parade in front of us.
I think its all subjective though, you see such a variety of tanks on here with happy axolotls in, so be creative, think 'bottom of a lake' and you should be fine.
As far as sand goes, I find it pretty easy to clean, but like Chris and Carson say, its a personal preference. I like having something to root plants into. Currently I'm having a lot of success in the tank with lilaeopsis, have java moss tied to a fake root, and have quite a few anubias and java fern tied to things. Plus, I'm (slowly) growing a moss wall.
The axies love the lilaeopsis, but it does root in the sand so you'd need a substrate, otherwise anubias and java fern will tie to rocks or wood.
Marimo balls are pretty funky too!