I'm not super experienced with networking. Does this require BGP for the router to move between hosts?

If my PON is connected into one of the nodes, how do I allow the router to roam if a node fails? Will look into openvswitch.

Not really ever done virtualized networking so it doesn't immediately click for me.

Put your PON on a separate VLAN on the switch

Ok, It's slowly coming together. So I assume I'd then put the virtual WANs of each cluster node on the same VLAN and whichever is currently hosting the router would chat with the PON to give me a WAN link.