I personally just change out a larger toy at a time occasionally,and clean the cage(80 cm x 50 cm temporary bin setup,will be larger and then be even more enriching and more suitable for my Syrian)every 3-6 weeks halfway.While it is the "same" substrate,you're not going to be doing anything healthy by giving hamsters completely new substrate every week or so.That would actually be counterproductive as it is stressful and would do more harm than good and could also cause illness or put hamsters more at risk for parasites and such
It is however fine to do things like adding a new larger toy on occasion instead,or switching these out.Smaller toys and things that aren't so intimidating can be given or changed more frequently,such as having a few different types of chews instead of just one or two.
It's not necessarily true that bigger is more enriching automatically,and that's just common sense I suppose.Think of it this way though,lots of enrichment and minimal open space.A cage could be a square meter in floor space and it would still be boring until there was proper enrichment added,but that being said there are many different things involved with enrichment that if combined do far better than having one or two on their own such as having a large amount of floor space,having a good and varied diet,having lots of enriching chews and toys,deep substrate that is appropriate to burrowing,etc.