Hi,
okay, thus if I understand it correctly, then you entered each calendar
manually with File->New->Calendar.
Try to use File->New->Collection account and fill in the Nextcloud URL,
ending with …/remote.php/dav/ . That will add you all the calendars
from that server in one go and they will be grouped together in its own
group, not under the “CalDAV” group.
Bye,
Milan