The danger with Bare Bottom tanks is they can build up a coat of harmful bacteria. Fish breeders are finding, they need to wipe the bottom daily or they were loosing fry.
Big rocks or marbles allow food to drop between and while you can vacuum them its not ideal.
Smooth gravel at least 1" deep will grow beneficial bacteria. You can vacuum it, one section a week. Or use a shallow layer.
Sand works well for me. Debris sits on top, I can siphon it out by hovering the hose above it without removing sand. You can do a shallow layer or go deeper. As long as its less than 3" you shouldn't have issues. I use Malaysian Trumpet Snails to aerate and clean my sand.
I have live plants, so I enjoy my black sand tanks.
Yours look nice. Enjoy your tanks