Comment by MaKey

You could set up a monitoring solution that alerts you if one of your devices is suddenly reachable from the internet via IPv6. It will probably never fire an alert but in your case might help you sleep better. IPv6 privacy extensions could help you too.

In practice I don't think it's really an issue. The IPv6 firewall will probably not break in a way that makes your device reachable from the internet. Even if it would, someone would have to know the IPv6 address of the device they want to target - which means that you have to connect to a system that they have control of first, otherwise it's unlikely they'll ever get it. Lastly, you'd have to run some kind of software on that device that has a vulnerability which can be exploited via network. Combine all that and it gets so unlikely that you'll get hacked this way that it's not worth worrying about.