I have OPNsense running on a 2nd-hand Dell Optiplex SFF, which I added 2-port NIC(s) to (initially Gigabit RJ45, then later 10Gbit SFP).

Currently it’s running bridged from my ISP router, but I’m in the process of wiring the fibre directly via a SFP transceiver to the OPNsense box.