I'm sure there's better options now but the HP ProLiant MicroServers (used).

They support ECC ram, 4 caddies, one extra PCIe slot, and to my knowledge you're not cpu limited for a zfs file server usecase.

Keep in mind though, all you need is linux* support, iDRAC, ECC if you're a snob, and drive bays ... and that's basically any free server.

In my extremely opinionated opinion I would only get used enterprise server gear, because a zfs file server will just work unless hardware fails. And a UPS.

*ZFS will be a more natural choice on FreeBSD. It's better documented, and will meet Linux 1:1 in hardware compatibility for this.

Not strictly a recommendation, but Terramaster is a good brand to look at if you want Synology-shaped hardware which can run TrueNAS or Proxmox or any flavour of Linux you want.

Along with various other devices (including a large Synology which I wouldn’t buy today), I run Proxmox on a small two bay+two nvme Terramaster. I have a bare bones Ubuntu LXC running Samba configured for Apple Time Machine, an VM running Scrypyed, and PBS for Proxmox backups. Nothing on it is critical so I don’t bother with any storage redundancy.

It's a horrible idea likely, but I have an ancient old Dell PowerEdge R510. Probably sucks way too much energy, but it does what it does and the price of SSDs have skyrocketed so I'm not touching it.