I’ve been advised elsewhere that this is not a good idea for a first attempt so I will follow their advice and drop this idea.

Foldingbikes.co.uk can make me a set of 20” 406 wheels with 20h and 28h using Velocity Razor rims and Hope Pro 3 hubs but that rim is very narrow at 20mm and I would be concerned about difficulty in fitting and removing tyres. Would prefer Aeroheat rims but can only find them in 32h and 36h.