Everything seems to be working fine until I'm restarting the opnsense device.
A running ping (from another client through the opnsense) on IPv6 targets works, of course runs in a timeout during reboot, but stays "request timed out" after reboot. Stopping the ping and re-initiating the ping doesn't work.
For now it seems that the routing is getting restored after 10-15 minutes.
That is not normal behaviour. Can you still ping IPv6 targets from the firewall itself?
Does your provider give you a static IPv6 block? If so, it may be worth picking a /64 from that range and assign that to your LAN instead of tracking the WAN interface.
Also check the DHCPv6 client configuration on the WAN interface page. Particularly try the 'Use IPv4 connectivity' setting.
Sorry, missed your line about the routing being restored after 10-15 minutes. That is a bit long for the interface to settle down but not too unusual.