If you do not want VLANs then using the "dumb" 2.5Gbps switch will still be performing a whole lot better than trying to use your OPNsense as switch. But if you really insist: https://docs.opnsense.org/manual/how-tos/lan_bridge.html
Couldn't be simpler. If you connect a Lan port on your OPN to a port on an unmanaged switch, then all the other ports on that switch will be on the same LAN.By default each port of your N100 is a different network and anything you plug into those ports will be isolated from each other.