alt Hacker NewsI 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.
Are you sure? I've seen plenty of motherboards bundle a PCIe riser to passively bifurcate the PCIe slot to support four M.2 drives in an x16 slot or two in an x8 slot, but doing eight M.2 drives in one PCIe slot would either require a PCIe switch that would be too expensive for a free bundled card, or require PCIe bifurcation down to two lanes per link, which I don't think any workstation CPUs have ever supported. And 32TB is possible with just four M.2 SSDs.