I haven't read about saucer style wheels hurting backs, so i can't comment on how much of an issue that is/if it's an issue at all. But I do know that animals running on a circular track (such as that of the flying saucer) can develop joint and muscle problems because their bodies are being worked unevenly; it's also seen in racehorses who aren't worked in both directions of the track.
For upright wheels I'd suggest looking into the Carolina Storm wheels
or TicTac wheels
. I don't know where you are, but the latter (TicTac) ship worldwide.
I adopted/rescued my hedgehog from someone who didn't look after her properly, and they gave her much too small of a wheel - so she had no idea what a wheel was when I took her home. It took her a while to get accustomed to it/realise what it is, but within 2-4 weeks she was a pro. I didn't do anything special; I just left it in and she sussed it out on her own terms.