Some of the models do - I have the M720q and it has a small form factor PCIe 8x slot on the motherboard, but you need a proprietary riser board (cheap on aliexpress) to convert it to a real PCIe slot, and it takes the place of the internal SATA drive cage and rear port adapters (VGA or serial or what have you) so you can't use those.

I'm using mine as an OpenWRT router with a dual 10 Gbe ethernet card in the PCIe slot ever since I got 10 Gbit internet at home and consumer 10 Gbit routers are expensive and leave a bit to be desired

In theory you can use the WiFi-One, though I don't know if that one is BIOS-locked. I just use it with USB3 and a 4 Bay Enclosure, which probably someone is gonna explain to me is the worst idea since leaded gasoline, but I have absolutely no Issues with my ZFS NAS for 5 years that way.

The easiest way might be getting an ITX board with N100/150 from CWWK or Topton. They usually come with 4-6 SATA ports and the CPU soldered on. Then you just buy your favorite case and plug in your drives. Excluding the cost of the drives you might end up spending about $250, depending on your case and PSU selection. The problem with using SBCs or mini PCs that way is that you need an enclosure to hold, power, and cool down your disks, and even that is assuming that you can actually connect them via a SATA/SAS connection which may not be easily available from within your case.

Check out Hardware Haven on Youtube, he tests a bunch of different small form factor NAS computers and similar. I find the whole "raspi ecosystem" a bit overrated, for the most part it's Linux you're working with regardless of hardware.

If you go with x86 you've got a bunch of people doing it under the "homelab" banner, so you'll hardly be alone.