Quick heads up: the mwan3 package (used to monitor and load balance between multiple WAN interfaces) does not appear to work well with IPv6. Here’s a bug report on it.
I was re-enabling IPv6 on my router and found that I couldn’t ping my upstream gateway, even from the router itself; pings failed with “ping6: sendto: Permission denied” or “ping6: sendto: Network is Unreachable”
Removing mwan3 solved the issue.
There’s probably a configuration setting I have overlooked that will fix the issue, but for now, I just want my ipv6 working again!