Ethernet bridging Mac Mini
Mostly, you can just overload the one ethernet hardware port with two or more IP settings. As long as your switch can handle things, this works the simplest.

Or you could drop an thunderbolt to ethernet adapter on the back of the Mac Mini and have two hardware interfaces to let the Mac Mini sit between the two networks and route as needed. You would run OpenVPN on the adapter en0 which connects to fiber.

All of this is controllable over the normal system preferences for networking but I've been able to accomplish all of what you describe without needing the unix command line. Just drag the default route interface to the top of the service order listing and you should be good to go.