Sorry for the late reply. The optiplex is absolutely sold as a desktop, but I’ve been using it as a server for a couple of years. Very small, quiet, and it sips power, but it only has support for 2 2.5" SATA drives (though it does have plenty of USB 3 ports if you don’t mind using external storage). TrueNAS installed on it would be similar to your Synology setup (without the vendor lock in), though not nearly as convenient as the front bays you’re used to.

It’s yours if you want it, but no worries otherwise.

That would be awesome. I have a Dell Optiplex 7040 Micro that might be better suited for your needs after you play around with the big server. Maybe we could trade?

Depending on where you’re located I would happily pay for shipping.

If you don’t want it, I’ll take it off your hands. I have a rack designed for that kind of hardware and all of the infrastructure to fully support it.

I would recommend editing your fstab and use the USB drive’s UUID to mount it to a consistent location. That way, even if you reboot or disconnect the drive and the reconnect it to, say, a different USB port, it will always mount to the same mountpoint.

See this page from the arch wiki for more info.