hi,
i would have the same issue here, where proton's service works great with wireguard on the router (owrt 19), but impossible to have ipv6 shared.
question would be : proton free doesnt provide prefix, so this kind of thing is supposed to be fixed?
https://openwrt.org/docs/guide-user/network/ipv6/ipv6.nat6
where a /etc/config/wireless & networks, without uci, would be appreciated 
if someone have the solution to get ipv6 from wg/proton... 