Wheelchair section of the Schwalbe catalog is interesting - also started a youtube adventure on the exploits of Aaron Fotherinham who does flips and rail grinds in his.
Seems like 559 is one size of bike overlap.
It you can split the problem into hub & bearings, rim with a common bike BSD that already has the hand rail, and a bike source tire, then provided you're willing to build wheels (fun learning project, took a long evening) then there could be a fair degree of supply independence and maintainability?
Have you determined current rim BSD is not a common size with multiple tire suppliers? If it is, maybe you can save the rim, and just replace the hub. Custom length spokes should not be any great issue or expense to source as long as they're less then about 300mm, and there are calculators online that work if you have accurate rim and flange measurements.