I bought my 990FX back in 2011 so no M.2 SSD but at least a 2.5-inch SATA SSD works well with it.Of course you shouldn't have any problems with XP on a 990FX system. AMD's 990FX platform is completely supported with full WindowsXP Support for all onboard devices on all motherboards. It's an extremely slow of a system in actual performance. But it is supported.
I have a question about you using an SSD with Windows XP. Since it doesn't support TRIM can I assume you have done the following:
- Disabled defrag
- Have a TRIM and SSD optimizer program
Reading your success if I find time I might try Windows XP on my 990FX chipset system just to see what it can do. I just have to be careful because other drives have Windows 7 and Windows 10 on them. From past experience I have found that Windows XP does not play well with Windows 7 and Windows 10 so I have to be careful it doesn't mess with the drives they are on.