I admit I didn’t even know you could just buy a mobo with CPUs and pretty much build it like any regular PC https://www.raptorcs.com/content/TL2B02/intro.html
And you can, more or less, use you PCI GPU too https://wiki.raptorcs.com/wiki/POWER9_Hardware_Compatibility...
There is a prebuilt version too https://www.raptorcs.com/content/TL2WK2/intro.html
On the other hand what’s the use case of a POWER9 desktop system? I’ve only heard about them in supercomputers and mainframes but why would anyone use one as a, daily, desktop PC?