Thanks for the forum praise 
Of course I didn’t expect that you’re living in China, the closeness to Taiwan and Korea will make things much easier and cheaper for you naturally.
At least keep an eye at the surface of your freewheel; rain in combination with street dirt will act like a grinder and the teeth will get smaller (the more riding in rain - the sooner). The result is lower belt tension, belt skipping may occur. Under bad conditions it is no problem to wear out a freewheel completely within 1500 kms.
I think it is a price/niche market thing.
From Europe you can order a complete 16" standard wheel for maybe 25US$ (plus transport and customs), the local price for an original Strida wheel will be around 50 Euros.
And just a few people are in need of a special small wheel - which would cost about three or four times as much in the end…
At the end of this post you can find an cost estimation for DIY wheel sets (using standard parts), already over five years old are the amounts still matching.
Btw - I do know just one company which is making Strida custom wheels professionally -
http://koreamtb.co.kr/shop/list.php?ca_id=40&sort=&sortodr=&page=1
maybe you could talk to them (Korean) about your project?
This shows the dimensions of the axles (upper = front); you may need it for designing a hub.
