I'm sure you would want flesh memory nowadays.
Asus has that one all-m.2 NAS but it only has 4GB RAM and not much in the way of flexibility.
I'd look at a $1500
Asrock Rack ALTRAD8UD-1L2T paired with a 40W Q32-17 (literally, 32 1.7GHz Ampere cores). I'd set it in a 2U rack enclosure or perhaps an older version of the Silverstone LaScala, which can use pair of 4-in-1 5.25" U.2 backplanes ($400 each), probably with a high-efficiency 600W-ish PSU. Not sure how much RAM I'd put it but 8x32GB RDIMMs ($300ish) seems reasonable for a TrueNAS system. There IS a passive heat sink for at least some versions of Ampere, although in that case I'm sure I'd want to pay some attention to fans in the rest of the chassis.
Used ALTRA8UDs sell for about $900 including a CPU, albeit not usually the low power one.
That motherboard has native support for 4x SlimSAS, which is good enough for eight NVMe drives, and it has a pair of oculink ports (potentially two more). It has dual native 10GbE, four physical x16 PCIe slots and two m.2 drives and it an Aspeed controller for BCM goodness.
So there are potentially 10 ways to add NVMe to that thing without even involving any other HBAs, but if you wanted to add a faster NIC and an HBA for your spinning drives, go for it.
I don't see any 8TB u.2 drives as cheap as $400 like I did at the beginning of the year but I do see some for around $650. It's entirely possible that someone really trying to could get a 64TB all-NVMe or perhaps 4x15TB ($1200 each) with a crap-ton of room for expansion.
That is a setup that could be very future proof with lower overall power utilization that's probably lower than the average gaming PC.
A more realistic version would just be to get a Ryzen "GE" CPU. Ryzen is bad at going to low power states like the N100s, but a Ryzen 5700GE is a 35W part with 8c/16t that could live on an x470 or x570 board with 128GB RAM and enough I/O to take a fast NIC and HBA, and one of my nifty little 4 drive u.2 adapter boards. Is 35W low enough? Probably, since it's unlikely that you'd hit any of the CPU cores hard enough to make it boost with a NAS OS. There aren't really any weird Ryzen desktop boards with a mobile CPU on them the way there are for Intel. I did check.
Intel has ULV CPUs, but those on desktop boards generally max out at 32GB RAM and usually only have one x16 slot or x1 slot, and they don't ever seem to get paired with larger than ITX boards. Some of them do use MiniSAS instead of SATA but you'll never see one with 10GbE + x16PCIe + a large enough number of supported drives to make an appealing NAS.
The Minisforum MS-01 can run off an i5-12600H with roughly the same power envelope as the 5700GE. It has 2x10GbE, a x16 PCIe 4 slot and 3 m.2 drives with the option to install an extra back plate for four more m.2s. This is a little more interesting for a $600 PC, but it's still pretty light on drives IMO. I do suspect somebody has probably come up with a 3D printed bottom for that thing that can turn one of those m.2 slots into 6 SATA bays, and that's the point where I'd take the time to investigate one.