I recently discovered the same, which is true for SATA SSD drives, too. The 2.5" 2TB SSD for NAS are exactly $400 now. 🤯
I'm replacing my Synology with a UNAS, so I at least get to keep using them and have a few left over for replacements if I ever need them. And as I had hoped, the NAS SSDs have lasted forever compared to the 2.5/3.5 spinners that make too much noise.
@jon Ill have to take a look at UNAS, looks interesting. I too have Synology.
I have a stack of older used 1-2TB NAS drives and would not really work with my current setup, too small. Still, its good to have on hand just incase.