OK, cool. Yea, recall wan6 is dynamic, so I cannot select a SRC for the /128 route on advanced tab.
The only "static" thing is the link-local. I have to trust the ISP never swaps out the far-end PHY, which would change the link-local IP for the route; but that's OK because I assume if the neighbor drops, the 6in4 route takes over as you agree. When I notice, I can fix it. I likewise trust the 6in4 IPv4 endpoint will remain the same, this is no difference - in perspective.
It seems a more specific route for the /128 is the only solution in this WG use case with a specific known static remote endpoint peer IPv6 address, with Native IPv6 and a 6in4 tunnel locally. As @slh noted - you would use the CIDR of the entire subnet of the datacenter, etc. if you weren't sure of the single /128 IPv6 address (but...in that case, the remote end would need a static method of reaching local..which I noted was dynamic...meaning DDNS for IPv6 services...so 
).