NextCloud: unusable CardDAV-Sync with contact groups

I want to sync a large number of contacts in 85 groups with iPhones/iPads that I created in NextCloud. It's bad enough that I can only do contacts management on Mac with Thunderbird and not with the Contacts App, as the sync doesn't work there either.

On iOS it becomes completely absurd as the sync only captures the flat "all contacs" list, existing or new groups are not synced and contacts cannot be assigned to groups.

This makes using a large number of contacts on the iPhone completely impossible.

So my question, has anyone ever managed to sync contacts structured into groups with Nextcloud (Version 20.x) on iOS or macOS? If so how?

Primary discussion here: NextCloud: unusable CardDAV-Sync with contact groups