He's lovely
Looking at the photos he could maybe do with some more/bigger floor toys at the end away from the house. Things like cardboard tunnels go down well (pringles tubes are a good size for syrian tunnels) or a tissue box hide - places to go and explore.
I wouldn't move anything again as that might unsettle him as well, but adding a few new things at the emptier end would be accepted find - it;s just things being moved around they don't like.
He will need a bigger wheel as he gets older. A popular inexpensive one is the 28cm plastic trixie wheel (the one with holes in) - costs about £9.
Yes it may the cleanouts that are making him bury the house - to try and keep people out or something! They tend to see a sand dish/bath as different from a toilet. If he pees in a corner of the cage then it could be a good idea to add a corner litter tray with sand in there.
I use these.
Trixie Corner Toilet, 16cm x 7cm x 12cm: Amazon.co.uk: Pet Supplies
They seem to like corner litter trays. Ours are very tidy and pee in the back corner and sit in the front part to have a wash. You may find you don't need the sand bowl as well if that happens. You could try moving the sand bowl to the pee corner but he might pee over the edges if it doesn't fit properly in the corner. Or he may decide it's not a toilet because he's been using it as a sand bath!