interface WAN1 (actually a gigether)
ip unnumbered loopback0 (or no ip addr?)
That's not correct.
Actually, was fixating on the fact that he has ethernet WAN and last I
checked IOS doesn't have unnumbered-to-ethernet capability (well not true,
recently they introduced unnumbered SVI support, don't know if it actually
works on anything else)
it works perfectly well with
ip local interface Vlan2
where vlan2 is the wan interface on an 1812, for example.
You want to transport the real physical ports (fas0 or fas1 on the 1812)
to be able to transport vlan-tags, spanning-tree-bpd's, etc...
thru your l2tpv3 pseudowire,
so you will use one of the switch-ports for your "internet" Connection .
So there is no need to re-configure your working WAN connection.