TrueNAS was able to detect 4 NVMe m.2. Pre-req is that the mobo supports 4x4 PCIe bifurcation.
You said you need a lot of storage, not craploads :).
Bigger case might be a better solution as you intend to do, though there are some options that can be done to expand the storage slots of even that mini guy. You could use the dock and the space thats intended for the external GPU to install a pcie to nvme card, which will get you 4 more nvme slots. Also the USB ports to which you could connect USB hubs and external drives. 1 USB 3.2 Gen 2 could serve about 5 HDD's(or more slower ones) up to their full speed(2.5G nework card would be saturated by just one or max 2).
I think i covered all posibilities. GL with your project.
If you have any pcie slots left there are adapters for more m.2 slots. Asus makes one, as does Konyead. Note that these are x16 and therefore will split a x16 slot into 4 separate x4 m.2 slots, they will not split a x8 lane into 4 x2s or a x4 to 4x1s. So if you use a x8 slot only the first two m.2 slots will work. Also, your motherboard must support pcie bifurcation, a lot do, but some don't.
yes. You have to find one that's explicitly compatible. I couldn't tell you the technical details of how it's differentiated, only that both users and sellers have related that Gen4 must be specified on the card. (e.g. Asus sells a Hyper x16 that's Gen3 that's a separate listing from the Gen 4; it's similar to how riser cables must be listed Gen4 to support Gen4 speeds)
Depends on how many you want to add and how fast. gen 3 or 4?
https://www.amazon.com/ASUS-M-2-X16-Expansion-Card/dp/B084HMHGSP/ref=sr_1_10?crid=16CJHQICCTZA3&keywords=pcie+4.0x4+m.2+board&qid=1679622431&sprefix=pcie+4.0x+m.2+board%2Caps%2C488&sr=8-10&ufe=app_do%3Aamzn1.fos.18ed3cb5-28d5-4975-8bc7-93deae8f9840