Elite has a few models where the tire drops onto the roller, instead of the roller coming up to the tire, so that it's compatible with various sizes without needing an adapter or other adjustments. It simply pivots by itself to fit. The specs says to 20"-29". I've never used it though, but it would seem to work with 20".
https://www.elite-it.com/en/products...igital-smart-b
https://www.elite-it.com/en/products...ers/qubo-fluid
https://www.elite-it.com/en/products...er-mag-smart-b


There is also a similar design from Bkool. Specs also say 20-29 inches.
https://store.bkool.com/en/44-bkool-smart-go.html