Nice job skipping the explicit qualifier of "plain vanilla"? Being able to build your own version on top isn't the same as an existing tested product.

Not sure what you mean by "on top". All you need is to configure not just a single endpoint in each node's wireguard config, but all of them. That's still as vanilla and as "tested product" as it gets. It's just a regular wireguard configuration.

You can do any to any just fine [open NAT ports, run your own distributed fallback network of TURN/STUN relays, add the adequate routing entries to your routing tables on both sides, exchange certificates, all of this for every extra N connections], you just probably don't want to do (and then fix if it doesn't work or stops working) that if N is too big.

Yes. I may also run it on top of IPv6 and not care about all that, except for public key distribution.

IPv6 when it works is awesome.

However, outside some well-connected datacenters with multiple peering exchanges, I have no clue where in the world you can run everything in IPv6 with even a single nine in availability.

On my home I would say assuming single nine availability of IPv6 traffic is too much availability - it's very common that IPv6 is borked for several months in a row.