Wow, there was a ton of great discussion while I was traveling yesterday!

And hungba your cages are GORGEOUS, would you mind if I private messaged you to ask about them? They're exactly the sort of setup I would like to make/have.

I do keep my crested geckos on soil and I can see where making a "sticky" soil (The sand, soil, and moss mixture mentioned) is important. A packed surface that forms well is less likely to get thrown absolutely everywhere and provides a better texture for digging. I've also seen setups for some other lizard species where they are provided a "sandbox" (With other substrate of course) for their digging instinct, while keeping the rest of the enclosure on a humidity-appropriate substrate. I've also seen water dishes set on very short platforms of slate to help keep dirt out; the plus side is if the platforms have stable legs that go to the floor then the reptile can dig under it too.

All of that said I find the soil thing can be hit or miss. I love it for my cresties; makes cleanup a breeze and helps the humidity. I did NOT like it for my snakes and keep them on shredded aspen or cypress mulch. I have never heard of "pelletized" aspen, I'll go looking for it. I would love something finer. I like that the shredded stuff compacts into a "floor" and is easy to spot clean, but it's awfully large and sharp. It also isn't really ideal for my younger reptiles.