In the absence of a radiused uke for the moment, why not try some different strings, in combination with hand stretches and smart practice?
As skill on an instrument develops, one can usually play in more efficient and effortless ways. What seems hard now might be less hard in a few months with dedicated practice. Also, hand flexibility is something that can very much be trained, just look at guitarists and pianists. I realise you are dealing with arthritis but I believe the latest research argues that frequent exercise is a benefit.
A Timms has relatively high frets (which might help) - do you have one? I can't remember.