I got a small thinkcenter a while back that I currently run some docker stuff on (homeassistant, pihole, some custom stuff), and I am pretty happy with it. Do they have a spare pcie or something to attach all the drives to, though? I don't remember seeing anything like that when I was inside mine, though that was a while back, so my memory may be wrong

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.