Hello,
i managed to get installed XP on Z370 MB - https://www.gigabyte.com/Motherboard/Z370-AORUS-Gaming-7-rev-10#kf , but USB3 is PITA, only 1 Asmedia on board USB3.1 port are working. Intel USB3 is not supported.
I tried VIA 8xx USB and other Asmedia PCI-E controllers, but both are not working some error code in Device manager.. Sorry im not in machine location, but is something like Code 10 or Code 12, device is not working, on part of driver is not working.. Controller worked with other machine and are working with Windows 7/10.. so problem are XP drivers..
Etron and Renesas arent available outside of US on Ebay with reasonable price, what you thing about Fresco FL1100 its not cheap - 20 bucks? Or is there some other good USB3 controller manufactor?