Need advice on Upgrade Path - going from NC13/Zimbra to NC-Latest/LDAP

Hi folks, I’ve been using Nextcloud with the zimbra connector and really enjoying the functionality. I have ~100 users on Zimbra, some of whom are using the Nextcloud/Zimbradrive functions including the phone app.
As you know, this is now being discontinued by Zimbra and I need to update both Nextcloud and Zimbra to stay patched (NC is very out of date now!).

My current Nextcloud is version 13 running on a Centos 7 VM (VMware). My LDAP servers are Windows 2012 Domain Controllers running a windows Active Directory.
Users currently login either via Zimbra or by directly accessing the web interface and using their email address and AD Password (not sure why it’s email address and not username, but it works reliably).

So, I have the following goals.

  1. Disconnect Nextcloud from Zimbra (and then upgrade Zimbra to current version in a separate workflow).
  2. Enable LDAP on Nextcloud for user auth and provisioning.
  3. Upgrade Nextcloud to current version for security.
  4. Keep all the current files available to our existing AD users

It all looks pretty simple…so I took a snapshot and dived in. So far I have found that

  1. ldap user and group backend is installed but greyed out. It stays greyed out even if I disable the Zimbra Drive app
  2. Nextcloud updater fails with permission errors (unable to unlink files) that persist even after I chown -R the /html/nextcloud folder to the apache user.

So, I’m a bit stuck. What would you do in my situtation - fight the current nextcloud server through to nextcloud 17 or make a new VM and try to transfer the files? Does 17 even use the same directory structure as 13?
Thanks for any help, and Happy New Year!