FreeBSD that's what pfSense is also based on: https://pfsense.org
like I could even make an entire thread for all the options and possibilities of configuration

pfSense pretty much is an open source operating system to be the home router and can install it on literally any computer out there
fun stuff like setting up multiple WAN in LAGG, DNS over TLS on port 853/TCP rather than 53/UDP, wireguard server & / or client
can configure it through ssh but there's also a web interface, pretty neat