Sorry long long post. tl;dr 2 default IPv6 routes different metrics set up persistent on Debian.

I need a bit of advice concerning routing IPv6.
Here is the problem. I do quite a bit of travelling around and a lot of it in SE Asia. I frequently find my self where the relevant ISP does not provide IPv6 connectivity. Even here at home my connection will occasionally change my address or I have to reboot the router and get a different one. I do things over the net for which I want IPv6, but also for DNS I need a stable fixed address.
So I have an additional /56 subnet allocated to my VPS. Over the years I have tinkered with different VPN solutions to push these addresses down to my home network. I have found a different solution which not only was easy to set up, but works a dream except for one tiny issue.
The /56 has been added to eth0 of my VPS. I am running wireguard and have it set up interface wg0 to which I route a /60 subnet. <bitfolk prefix>:e10::/60. Packets hitting this are encrypted with the server key and then encapsulated in IPv4 UDP packets and sent to the wg0 interface on my home machine, decrypted and if meeting criteria move through firewall etc. Sending out it is the same in reverse, encryption being via the client keypair. The client wg0 has subnet <bitfolk prefix>:e10::2/64, the server only accepting packets from this range and properly encrypted.
Now here comes the problem. It is the default route issue. All that I read says that you cannot have 2 default routes in the same table. I have looked at a variety of solutions but find none except the one everyone seems to say is impossible but which works. I set the route
ip -6 route add ::0/0 dev wg0 metric 512.
Note the metric 512. The autoconfigured one has a metric of 1024. which gives me
ip -6 route
default dev wg0 metric 512 pref medium
default via fe80::42c7:29ff:fe26:78c9 dev enp3s0 proto ra metric 1024 expires 265sec mtu 1488 hoplimit 64 pref medium

When I have finished fiddling and checking I will change the wg0 route to metric 2000 so that traffic will normally go through the main interface and when that has no IPv6 connectivity or is playing up, the wg0 route will be selected, (I hope).
My 2 laptops, and Raspberry Pi will then be set up with their own wg1 etc interfaces and will then have their own /64 subnets.
But when I try to get the route established automatically through the wireguard conf files or through PostUp I get the message can't do it as there is already an autoconfigured default. So I am stuck, at the moment with adding manually after every boot/reboot. Any suggestions please?
VPS running Debian Stretch  This box at home running Debian Buster.
The only answer I can think of at the mo is turn off autoconfig, but then I lose this fallback mechanism and add difficulties with communicating with mobile phone/router etc. Or I guess I could forget the fancy fall back idea and just go through VPS but that could add a long delay when doing ordinary surfing. IPv4 of course just goes out through the normal interface