Today I've set up two OPNsense 26.1 Firewalls in a HA configuration on a single DSL internet connection.
Failover is working, the backup OPNsense does connect the PPPoE session, and requests a IPv6 prefix via DHCPv6, but it doesn't disconnected the PPP session after the main OPNsense comes back online.
Not sure why, I've set up CARP on the igb0_vlan interface, but it's not working. I probably forgot something, needs more troubleshooting.
Still, pretty nice not needing a router in front of the OPNsense Firewalls. :)