Hi Golden Syrian Hammies! I would just spot clean the substrate and pile a load of new on top then mix it in a bit
It helps cushion any falls from a height and sometimes they burrow in it too. Have you seen Erin's Hamsters 8 cage cleaning tips video? It's brilliant - it really helped me change how I did cage cleans - meaning you can have much deeper substrate but not actually using any more than if you chucked all the old and put the same amount of new in. It mainly helps avoid hammy stress as well as saving on substrate.
I also really like your shelves
These are just ideas, but with the substrate deeper you could have that lovely wodent wheel a bit higher - you can hang them upside down from the top of the cage. I did that when I had one and it makes the wheel spin much better I found, plus the wheel is more stable. You take the wheel off the stand, slot the axle part of the stand through the bars until the base is just sitting on top of the bars basically. Then you reattach the wheel to the stand inside the cage (with the bars off possibly might be easier but with a big front door on that cage it's possible to do it without taking the bars off). Then you just put about 3 cable ties on the stand to secure it to the bars - that makes it really stable. It means the stand stays permanently attached to the cage and you just take the wheel off the stand for cleaning. I used to just wipe round the inside of the wheel and only take it off for cleaning about once every couple of months (depending on if or how much your hammy pees in the wheel!). I'll add a link to show an example at the bottom. It's just a tip but I much preferred having a wodent wheel that way. It also means your hammy can use the floor space under the wheel and can access it easily with the deep substrate which should come to near the bottom of the wheel. If the gap between the substrate and the wheel is a bit high you could put a little bridge or ramp in front of it.
Because you have those nice shelves - something like a tube tied to the left hand side from that corner shelf, could give an extra run. I used to have a cardboard tube tied on in that way - with something like a hanging rat sputnik in the back left corner.
With something like that, your hammy could go up the bendy bridge on to the front left corner shelf, through the tube tied to the side and into the sputnik at the back. If he jumps out of the other side of the sputnik that should be ok if the substrate is deeper and there's nothing in the way to land on. It makes a whole extra run in the cage for them
and would make use of that space in the back left corner.
Anyway - those are just ideas to think about. I found another advantage of having the wodent wheel hung from the top was the stand is out of the way. When it's on the floor they can get a bit tangled up in the stand when running behind the wheel sometimes. Although it might mean having the wheel at the back left if you don't want to restrict access to the top door, instead of the sputnik. I actually had my 80cm cage set up a bit like that - wheel on the left back, sputnik in front of it near front left corner. You could tie a tube to the bars anywhere really as a route from one shelf to another or onto a house roof.
This is what my old 80cm cage looked like with a sputnik and tube (it was a fair bit lower than your cage though and a different wheel).
If you put some bedding in the rat sputniks they tend to like them as a cosy hidey place.
This is a Wobust wheel hung from the top (it's enormous! It's the 30cm one). You can probably just see there are 3 cable ties in strategic places. I found if I put more it pulled it so it didn't hang right - three just secured it so it didn't move about and could hang well).
And this is Erin's cage cleaning tips - the best bit I picked up was not cleaning everything at the same time - now I do partial cleans
https://www.youtube.com/watch?v=5jVnEjrXbww
Anyway they are just ideas for possibilities