It seems like it's rare to find M.2 with the sort of things you'd want in a NAS like PLP, reasonably high DWPD, good controllers, etc. and you've also got to contend with the problem of managing heat in a way I had never seen with 2.5 or 3.5 drives. I would imagine the sort of people doing NVMe for NAS/SAN/servers are all probably using U.2 or U.3 (I know I do).

When it comes to ready-made home/SMB-grade NASes, in recent year or two plenty of options popped up: Terramaster F8, Flashstor 6 or 12, BeeLink ME mini N150 (6x NVMe). It's just QNAP and Synology who seem not interested.

How well does buying PCIe to M.2 adapters work for a custom NAS? Slot-wise you should be able to get 16 M.2 devices per motherboard with for example a Supermicro consumer board.

I don't now if you consider it "reasonable" but the Gigabye Aorus TRX boards even from 6 years ago came with a free PCIE expansion card that held 8 M2 sticks, up to 32 TB on a consumer board. It's eATX, of course, so quite a bit bigger than an appliance NAS, and the socket is for a threadripper, more suitable for a hypervisor than a NAS, but if you're willing to blow five to ten grand and be severely overprovisioned, you can build a hell of a rig.