Im waiting for a solution as well. As mentioned above it seems pulling the spare off is the only way. Im sure someone has found a detail brush that fits in the space, I am still looking for one tho.
Kinda makes me miss the OG bronco's sway bar for the spare
I have this wheel cleaner tool, which I don't usually use for cleaning my wheels lol. It's great for fitting behind awkward areas, like the spare tire. So far I've just gotten it clean with a microfibre mitt the best I can, but over time this tool will be helpful.
If it's really bad, removing the spare tire is easy enough to do.
Microfiber towel a thin stick, like a ruler, and bucket of soapy water.
Alternatively, do what I did and look back there while washing and tell yourself it’s too much trouble and doesn‘t really matter since the truck is going to have tons of brush scratches soon enough….