Great to hear from you, and I hope that Michigan isn't under piles of snow and ice like a lot of the rest of the country!
Sphagnum moss should be absolutely fine to plant, and it won't harm Steve if he eats some. Here is our entry for it:
https://www.thetortoisetable.org.uk/pla ... DDhbXmnyM8
I think you might have to plant it in shady areas, as it could dry out in hot sunshine, but if it's native to your area then it should grow well.
I'm amazed you say that sedum is a plant that Steve wouldn't be tempted by! What a character he is. Most tortoises absolutely love sedum and will eat it to the ground. I once made a little tunnel for Doris out of a curved roofing tile, and thought I'd cover the top with soil and make it into a sedum roof (I think I used Sedum album and maybe Sedum reflexum). I put up a little barrier of log roll all around the roof so that the plants could take hold, and one day I came back to find Doris had barged her way through the barrier and had eaten almost everything that I'd spent weeks growing. Lol, I didn't try that again. Have attached a photo of the culprit caught in the act (it was not long after I got her, and her shell was still quite bumpy, which you can see in the photo, but subsequent years have seen really smooth growth so the bumps have almost disappeared). Maybe Steve is a more discerning tortoise and won't eat your sedum down to the ground at the first opportunity.
The only other thing I can think of that is fairly low growing is the ground cover type of Campanula (but again mine wouldl eat that in a heartbeat).
I'd love to see some photos of the enclosure when it's finished, and if I can think of anything else that would suit your purpose, I'll post it.