Well, I would rather reset your expectations. The topic that mwan3, for its IPv6 fail-over support, depends on something which is not in OpenWrt (unless you are OK with NATing everything to a single IPv6 address) has been raised before, but this did not lead to an understanding of a definite way forward. Please check these topics:
The official mwan3 documentation says:
Using mwan3 with IPv6 requires additional configuration such as IPv6 masquerading through methods like NETMAP or NAT6. This is currently not implemented in mwan3 directly and requires additional configuration. ... This is something that needs to be configured outside of mwan3 itself.
Currently, what's expressible using the core firewall features is NAT6, where all LAN hosts are masqueraded behind the router IPv6 address, just like it is done with IPv4. …
and
The current recommendation for WAN fail-over is to use mwan3. However, for IPv6, it needs NAT or NPT. This is understandable: mwan3 deals not only with fail-over, but also with load-balancing, and for load-balancing the decision regarding the choice of the source address must be delegated to the router on a per-connection basis, and therefore the originating host in the LAN must use a dummy temporary source address that will be replaced as needed.
However, none of that should be necessary if th…