Update:

Switching to Windows 10 Pro from the previous Server 2012R2 has mostly fixed this. 8 gpus work no problem now on both my Asus Z270-A and Asus Z-270-P boards. 9 GPUs on the Z270-A (using two M.2 adapters) still refuses to work though, it will boot into windows and then instantly lock up completely (cant move mouse or interact).