HowTo avoid administrative overload in groupware usage?

Currently I try to set up a NextCloud instance (V22.2.3 Enterprise), managed by IONOS, for the music groups of a church community.
Means, that about 300 musicians are participating in different ensembles, different communities, different voice groups, etc.
Of course, these memberships are not exclusive at all, e.g. some tenors will sing baritone, if needed.

The import of a file according RFC6350 works so far, but some properties, e.g. ROLE, are not usable. Somwhere hidden in space. Export the contact from NC address book brings them back into the produced vcard file.
Where can I find a log of the import, especially when errors are reported?
How can I use the multivalued ORG property (O=Magdalena, OU=MixedChoir, OU=…)
How can I launch a query and export the result into an other address book?
(And, it seems that address book entries and NC ACL users have manually to be synchronized.)

HowTo send email to all Magdalena MixedChoir Soprano singers? [(CATEGORY/Group EQals Magdalena) AND (Group EQ Mixed) AND (Group EQ Soprano)]
RFC6350 describes List elements and according membership properties. Could not find them.

How can I schedule a rehersal with Magdalena MixedChoir Soprano singers? Means invite them, wait for their accept/decline, see the answers and, if enough participants accepted, send a confirmation.

These tasks should be possible for choir leaders, and: we have no access to a NC command line interface. We are nailed to the web user interface.

Has anyone any hints to bypass these potential show stoppers?