I am considering using a low cost single board computer, with two ethernet NICs, to act as my router and bandwidth meter. I would like to continue using OPNsense as my firewall. Is there anything different to do in this use case, from the OPNsense side. I don't necessarily need the router functionality. I just want it to be a firewall with IDS/IPS.
Nothing different but an easier configuration
Make sure to have plenty of ram (and that fits). Since your using IPS, that can be resource intensive. Also you may want to check the NIC manufacture to look at compatibility. Realtek does not do so well with FreeBSD, in my experience.