That looks amazing
And I love those little china plates
If I might suggest the odd tweak for access it could be a bit easier for your future hammy. It's the little ladders leading up to the sand boxes. They are probably ok but with having holes between the steps and steep a hamster might struggle with that or feel insecure. It's very difficult getting access ramps sorted for some things though. A bendy bridge sideways would be an easier ramp for some of the sand boxes, but for others, you might not have the space for that on top of the platform - ie for the hamster to get round the side to get up the bendy bridge. What I ended up doing was making some little solid shallow steps out of rounds of cork bark in three different sizes. So the bottom step was a larger round, and the next two steps were smaller rounds. It was very easy to just stick the bottom edge of one round to the edge of the one below and I actually just used plastikote to do that and it held firm. The downside to that is the cork rounds were a bit expensive. I got them from Getzoo.
The steepness can make them avoid something or struggle with it as well, so the ladder against the clear sand box front left, could be an issue. To reduce the steepness of that, either the sand box would need to be lower or more substrate under the ladder - both of which are tricky! For that one I do think a bendy stick bridge sideways on would solve that issue. No need to lower the sand box or raise the substrate then.
The engineering is always very tricky I find! Working out the height of platforms for the eventual height of things, but rule of thumb is a level shouldn't be more than 2" above substrate height - or a second level would need a shallow-ish ramp. I also sometimes use a cork log alongside a platform and it then doubles up as a ramp as well as tunnel - they just run up the side of it, and although it's fairly steep, it's also grippy with the texture - but not quite as steep as the current ladder front left. A cork log could work in front of the front left sand box, but actually an easier option might be move your grass 3 way tunnel in front of the sand box so one end is over the house door, the short end is pointing out towards the middle of the cage - that would give a nice dark tunnel entrance for the house and the hamster could easily climb the outside of the grass tunnel to reach the sand box as it's grippy
Alternatively you could "bridge" the sand bath with the house roof - eg a flat wide-ladder spanning the house roof to the roof of the sand box and the hamster can just run across - or even a tube/tunnel - the difficulty there would be keeping it stable.
The other little ladder with holes back right, I don;t think the hamster would be able to get to to climb it as it's right at the edge of the platform and a bendy stick bridge sideways onto the box would work really well there. so the hamster could climb up from the roof and not have to go all the way round to the edge of the ladder - and could fall off.
If still using any of the little ladders it would be best to stick a piece of wood or cardboard to the back so a leg can't slip all the way through if the hamster misses their footing.
The corner sand box back left looks just a bit too high for the hamster to reach from the low little ladder. I'm not sure what to suggest there and will have a think about it.
Final thing - the dish under the water bottle, the hamster will probably tip over. Something flat there would be better, eg a granite tile or piece of slate - that soaks up any water drips and can't flip up on the hamster or be knocked off. I have sometimes used small terracotta plant pot bases under a water bottle.
I hope all that makes sense! It's not criticism by any means - it's a really lovely enriched set up - it's just from experience of hamster access! It helps if you don't have to start changing things after they move in and they can get quite flummoxed if they can't reach something or feel insecure trying to reach it.
I love that you have cork on top of everything
Ness
Ness