Phil, have you seen this board?

Might be worth ordering one for testing. Riser-less 6GPU rigs would save you lots of components from your 3GPU rigs.

Manufacturer page
http://www.onda.cn/MotherBoard_Designs.aspx?id=369

Translated by Google
https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.onda.cn%2FMotherBoard_Designs.aspx%3Fid%3D369&edit-text=&act=url

Specs:
https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.onda.cn%2FMotherBoard_Specifications.aspx%3Fid%3D369&edit-text=&act=url


List price is CNY 1100, but the manufacturer's web shop is out of stock.

However, I found it on this website
https://s.taobao.com/search?q=D1800-BTC

Cheapest seem to be around CNY 590, which is the equivalent of $90.