I will need PoE. Which is on the Router. So I am keeping the Router. I also have a network inventory via Ubiquiti UNMS that I use on the Router, with the Switch, not sure that can be done in OPNSense.
But please forget this "firewall in fron of router" nonsense. Which it is.
The ISP WAN cable goes into igb0 on the OPNSense machine. Then after that I have at least two options. Either I connect the LAN igb1 to the Routers WAN port, or, as Herr Pmhausen might suggest I connect it to the switch port1.
ISP ROUTER | OPNsense | NETWORKWAN -> |NAT| LAN -> WAN -> |NAT| LAN -> SWITCH -> COMPUTERS75.32.53.67 |NAT| 192.168.0.1 | 192.168.0.2 |NAT| 192.168.1.1 -> DHCP