Hi there. What size is your bin length and depth? Because tbh it looks fine to me! I wouldn't worry about having deep substrate in every part of the cage. It is often necessary (when fitting a 28cm wheel in) to have no substrate under the wheel and shallower substrate near it (I used to slope the substrate away from the wheel. As long as you have deeper substrate elsewhere in the cage. Some hamsters aren't even bothered by deeper substrate or aren't diggers.
It looks like a really good sized bin cage to me. So don't worry too much about the wheel area having less substrate.
The only way you could have the same sized bin or cage and still have substrate under the wheel, would be to have a taller cage/bin and have the wheel on a platform. Taller bins aren't that easy to find. A detolf is a good size, but again the wheel area has to have virtually no substrate as the detolf is quite low - although you do have a lot more cage space for deeper areas.
One bin that is seen to be a really good size for a Syrian is the RUB 145 litre bin. Which is 81cm by 62 cm - so has a good depth - deeper than most cages. It's 43 cm tall which "might" be tall enough to have a 28cm wheel on a platform allowing for the depth of the lid.
But I'm not sure it would give more floor area than you currently have.
Go Shopping - Really Useful Boxes - 145 litre Really Useful Box
The main thing is he has a good wheel to run in now, plenty of floor space and some depth of substrate in some areas. I think another important thing is a good house and possibly a shelf or platform - that really adds to enrichment.