The documented solution requires NAT6 and that requires copy and pasting a script from the wiki (which does not seem to be packaged) which looks like it causes extra work on upgrades.
And then, there is the next problem that the client does not know its public IPv6 address anymore. For me, IPv6 is more important than IPv4.
Providing multiple IPv6 uplinks to a (Linux) client with OpenWrt works out of the box with Router Advertisements which can inform a client about multiple public network prefixes. OpenWrt provides the correct routing out of the box so that the different source addresses are routed to the different uplinks. The only remaining problem is the source address selection algorithm at the clients that mainly uses the longest matching prefix.
To solve this, I am using a custom proxy server running at the client computer (not at the OpenWrt router) that manually selects different public source addresses to use multiple uplinks. One positive aspect of this is that this can be enabled per application.
Instead of masquerade, I was able to rewrite the prefix of the addresses. This must be repeated when the addresses change and I did not automate that yet. Interestingly, I had to restart mwan3 before it worked but I am not yet sure about the cause for this.
There is only one sane, non-hacky solution for multihoming with IPv6.
Get your own ASN and GUA IPv6 prefix, announce it to multiple transit providers over BGP, balance it the way you want.
There are costs, but these days it can be done for like 15$ per year, if you are OK with using tunnels for IPv6 connectivity.
AS-prepends, BGP communities... takes some tweaking.
Me personally, I only BGP multihome for reliability reasons, active-passive configuration
Perhaps your ISPs can provide you with BGP sessions (not very likely for consumer plans though)
Tunnels in your country can provide with a low latency as well.
If you are in Germany, test latency to these servers.
This does not sound like something that works out of the box without trouble
Does this need BGP at all? Outages are very uncommon and I would have no problem with a IP address change in this case.
With tunneling, I could rent a server and use a VPN to use the public addresses of the server. This avoids getting a ASN and using BGP. The round trip time to one of the servers I rented is comparable to one of the mentioned servers (16 ms).
While I think that playing with BGP would be good for learning purposes, I don't think that it is a good fit for multiwan using consumer internet connections.