You may also want to look into your DHCLIENT operations and utilize dhclient-script functionality.
https://www.mirbsd.org/htman/i386/ma...ent-script.htm
remember, you're the dhclient of your isp, so let the dhcp-client event trigger the script and then let the script pass the new gateway parameters and, if necessary, trigger a pfctl -F all -f/etc/pf.conf.
/S