After spending half a day with CARP, iBGP, OSPFv3 and "pfsync" (pf state table synchronization), I now have seamless failover for my datacenter router working 🙂
Legacy-IP switches to a different provider assigned IP (I don't care!) while IPv6 stays exactly the same, without even dropping the SSH connection. 🥳
That was harder than I thought, but the result is absolutely amazing! My IPv6-network now survives when one node goes down, and IPv4 just switches over to anther egress NAT on another node.