> I'd love to find a M2-only NAS in a very low profile form factor.

QNAP has at least two "NASbooks":

https://www.qnap.com/en-us/product/tbs-464 (2.5 GbE, USB 3.2)

https://www.qnap.com/en-us/product/tbs-h574tx (2.5/10 GbE, Thunderbolt 4)