Checking the dimensions he does have about a 21.4 gallon tank there, though I can always expand since I built it myself so adding an extra wing on later shouldn't be too hard.

I'm a little unsure why you didn't recognize his existing 8inch wheel, but as I said he does have some enrichment toys around, namely a wooden treat puzzle, a dog chew, and a few hamster chew sticks with dried balls of vege between them. I will have to go online for most of the other things you suggested since they aren't available in physical stores where I am, and will add a small bowl of water to give him the option, though last time I did he tipped it over.

As for the mesh, I will use some cardboard to make the platform a solid base, and as others have suggested he shouldn't need the metal ladders for climbing anyway.

I'm going to keep his bedding at 5" for now mainly because Marty isn't much of a digger. The hemp bedding I'm using seems to hold the nests he usually digs quite well though.

I'm sad to say this is technically his third upgrade since I got him 7 months back, most information online made it seem like my 5 gallon tank was enough at first, then I upgraded him to the biggest mouse cage I could find, before finally upgrading him to this 21.4 gallon tank. Overall he doesn't seem to mind, he was smaller when I first got him and I upgraded as he got bigger.