He has loads of toys etc in there
Does he have a house or nest under a shelf? I would be tempted to have a lot more substrate in - at one end at least so he can burrow down - maybe a deep end with the shelf from that end butted up to the shelf at the wheel end - so it makes one longer platform and beyond that deeper substrate with a largeish house sunk into it. They are always happier when they can retreat somewhere to a large dark house where they can build a big nest for winter (they do feel the cold) and have space for hoards inside it. Sorry I forgot you’ve had hammies before but each hammy and each cage varies.
I’ve used labyrinth houses for our hammies and they love them and always nest in the darkest room inside. Rodipet make great houses and labyrinth houses but they are expensive. Before that I had the Ferplast sin rabbit house with a bendy bridge over the door - way too small for a rabbit - a bit smaller than a labyrinth house and no rooms but it worked well because a corner potty fits inside. When you give them a large house they invariably move the pee place inside the house in the corner opposite the door and nest in the corner furthest away from the door. With a flat roof it also doubles up as another platform and has a lift off roof like the labyrinth houses so you can check inside and empty the potty without having to take the house out and the nest falling apart.
With the larger houses there is the danger they can tunnel under them and get squashed so they either need supporting underneath to allow for that (Rodipet actually sell legs for their houses) or go the other way and have it on the bottom of the cage with the substrate piled up around it and just a bit of substrate inside the house to make it more comfortable - so then it’s like an undrrgroind burrow.
For all there is a lot of enrichment on the cage I could see he might feel a bit exposed and coukdn’t Tunnel down to get in the dark.
Just a thought. Rodipet also do nice 2 or 3 room houses but also expensive! Or you could just make a shoebox house - cut the bottom ou and a hole at one end of a long side for a door and keep the lid as a lift off roof and put a bendy bridge tunnel over the door so it’s dark inside and makes a ramp up to the roof as well.
I’d also suggest something like a large log tunnel at the front of the cage in the middle - so he’s a bit protected from the open feeling at the front and can go through it from one end to the other.
Obviously making changes will unsettle him but I find it’s accepted well if you add something new that they like and feel happy with - like a large house.
I have the Syrian sized largest labyrinth house with the saucer in the middle- both our Syrians have loved this house. The plate lifts off making it easy to remove the potty without taking the whole lid off. But it is expensive and postage is now about £8 as well. And a home made one or something similR is just as good.
Anyway sorry if he has a good house already and I just can’t see it. Ideally they have one that’s open underneath and on top of substrate so they can feel cosy and bury hoards (or put hoards in a room or area of the house).
Maybe with some changes to make it cosier and more cover and some bathtub taming he’ll start to get more confident.
On the other hand I agree that too access isn’t so good for interaction and I personally prefer cages for that reason like the Slavic PlZa with a big front door. But it seems a shame when it’s such a lovely big Perspex cage. Having the substrate deeper though would mean everything’s higher up so less of A height thing from the top.