Hi Marco,
moving the discussion from: https://gitlab.torproject.org/tpo/core/tor/-/issues/41033#note_3181625 to tor-relays as the features in the issue have already been implemented by Nick and the discussion has evolved into a more general direction.
I think many people have HiddenService instances in addition to their relay instances. HiddenService instances must then be configured not to use default FamilyId * if tor automatically detect and use those.
Ultimately, @nickm has to decide how much work he wants to put into c-tor. (Especially auto detecting KeyDir on all OS requires a lot of time to test.)
Happy families is not relevant for onion services. I sense a misunderstanding of "FamilyId *". It only looks for family keys in the "keys" subfolder of a specific tor instance's DataDirectory or within the directory given in "FamilyKeyDirectory". See also the man page section: "KeyDirectory" because "keys" is just the default but can be configured. It does not search across the all "keys" subfolders of multiple tor instances on the same server, which would fail anyway with proper filesystem permissions, because one tor instance should not be able to read another tor instance's data directory if setup correctly.
kind regards, nusenu