Given the impressive specs that the Raspberry Pi 4 boasts -- https://www.raspberrypi.org/products/raspberry-pi-4-model-b/ -- Broadcom BCM2711, Quad core @ 1.5GHz; up to 4GB SDRAM; 802.11ac wireless, Bluetooth 5.0,
Gigabit Ethernet --
Would it be possible to have two of these boards working together, with one ETH port serving the WAN side and the other, the LAN side?
you can add usb-rj45 adapters and get a firewall even with a raspi 2, but it's no real fun. raspi 4 would be looking good, but is currently lacking BSD support (which won't change too soon, considering the lack of support for raspi 3 atm)...
The Pi has always had throughput issues with Ethernet due to the way the network hardware connects. If you need more than about 100 Mbps, you need to wait for the 3B+ or the 4 model to be supported by FreeBSD/OPNsense.
If your throughput requirement is less than 500 Mbps, you can run a LAN and WAN VLAN trunked through any single NIC Gigabit device. For an older Pi this is around 50 with VLAN's or a bit more if you can find a working USB ethernet adapter with decent througput
Bart...
How to setup LAN and WAN in a single ETH port raspberry pi 3 b+
Separate VLAN's