Is this for a dwarf hamster or a Syrian? A 12” Syrian flying saucer wouldn’t fit on top of the Rodipet house and be secure. A 6.5” dwarf hamster saucer would I think.
When I had that cage (it wasn’t actually the hamster heaven but similar Savic cage same size) I had it with none of the shelves or plastic stuff etc. Just had the large house on one side and the wheel in the opposite corner at the back.
If it’s for a Syrian they do need an upright wheel no smaller than 8” diameter and 11” ideally. They can have a saucer as well but it needs to be the 12” one so both would take up a lot of space. For a dwarf hamster then a saucer wheel is fine - they run differently in them being smaller, and the 6.5” saucer is fine but they also enjoy an upright wheel as well
Anyway if you take the green shelf out - the one with the holes in - as well, you have room for the wheel