Buy something 240GB or bigger. SATA / NVME, it doesn't really matter just so long as it has DRAM on the SSD. Just but a decent brand name but cheap is fine. This would be great for you and cheap too.
Install Windows on the new SSD, leave the existing drives as storage. Sit back and enjoy your radically more responsive system - seriously this will be sooooooooooooooo much better.
NVME, M2, etc, it's all just diminishing returns to be honest.
Is dram cache that noticeable? I got the cheaper crucial drive not even realising it was a thing and the stutters I get when writing bulk data to it are horrific. The machine is almost unusable just in windows whilst downloading or copying from another drive, I don't experience the issue doing it to a second mechanical drive so I'm confident my SSD is the thing at fault here. I pulled a stick of ram from the pc I use a server and it's makes the main pc work nicer but not fixed the stuttering problem.
I was looking at getting a usb SSD for my Xbox one anyway so I might just buy the 500gb crucial drive, swap it out with the one in my pc and stick the one without dram in an enclosure for the Xbox. I'll also consider sticking another 8gb kit in the old thing as that's actually improved performance far more than I thought it would.