I would like to force anyone that plugs in to my "dumb" wired switch to be forced to authenticate through the configured FreeRADIUS server. I have FreeRADIUS running in OPNSense to authenticate WiFi, but the AP is basically a smart switch configured to look for the RADIUS server and authenticate through WPA-Enterprise 802.1x. Is it possible to do the same natively through OPNSense without a smart switch? All the pieces seem to be there already.
Short answer: no.
Longer answer: how should anything on the firewall prevent two devices on your dumb switch from talking to each other? The point of 802.1x is to prevent layer 2 network access completely. That must be done by the switch.