I've heard the Flashtor 12 is great, and can take lots of M.2s, but they end up with relatively few channels so data transmission rates aren't awesome. It can be fine as most users will be connecting over the network anyway which will be the bottleneck.
https://wiki.friendlyelec.com/wiki/index.php/CM3588_NAS_Kit
there is also the asustor nas: https://www.asustor.com/de/product?p_id=80
If you just need a single nvme SSD you could also buy a banana pi 3 Router:
https://wiki.banana-pi.org/Banana_Pi_BPI-R3
Edit: forgot links and product