Have a look at https://openbsdrouterguide.net/ https://openbsdrouterguide.net/ It is a very comprehensive guide to what you are planning to do. I have used a R

Have a look at https://openbsdrouterguide.net/ https://openbsdrouterguide.net/ It is a very comprehensive guide to what you are planning to do. I have used a Raspberry Pi with a USB dongle ethernet as the second NIC and FreeBSD (couldn't get OpenBSD to work). In the past I have also built almost as secure configurations using an old IBM ThinkCentre and an extra NIC plugged into the PCI bus, but with Ubuntu Linux. The openWRT project might be another option, but I don't have any experience with that.