servethehome.com has a series about these fanless, multi-gigabit firewall for a while, might be interesting if you have a 200-300 USD budget?
https://www.servethehome.com/tag/firewall/
I’ve used a very similar setup in the past (J1900 CPU, 4x1 Gbps network ports) and I only replaced it due to reasons. Not noticed any performance bottle necks with that setup.
The latest N100/N200/N300/N305 CPUs from Intel looks really interesting, similar performance as my workstation but at a 10th of the power usage. N305 also has 8 cores in a passively cooled case, amazing stuff!
I’ve thought about it, and nobody will care about your/my elaborate setup after we are gone. It will just be replaced by a ISP router without regrets.