vigorio I don’t think that your statement about “both method A and B required at the same time” is correct.
If you employ method B, then you configure all containers to use the nameserver you explicitly specify in docker-compose.override.yml. So, as long as this is not mailcow’s Unbound container, it will be completely irrelevant if you reconfigure mailcow’s Unbound. It will simply not be used for name resolution by mailcow’s different containers.
I felt I had to write this, as you had confused me, and I would like to prevent others from becoming confused as well. (I was struggling with frequent failed to query external DNS server errors from Docker, and researching this brought me here…)