Greetings,
If you didn't start with a m.2 drive on this board, don't bother investing in one now... yes, old tech and you'll be throwing money away.
NVMe is not supported, correct... once again, don't bother with an adapter. You will never attain the full speed potential of the drive.
Just buy an 850 EVO or Pro. depending on your size requirements and budget. Simply, easy, fast and very affordable now. This is the best cost to value ratio for this architecture.