You could (theoretically) setup an “external” storage location which is really an SSH tunnel to the Nextcloud folder location that is just local. This requires you have SSH access to a dedicated server and you’re not in a shared hosting environment. On your own server, you must also have it locked down with some kind of mitigation methodology to protect against bruteforce attacks (eg, fail2ban or some other method, configured to look for NextCloud login attempts-not just the NC bruteforce app), and layer yourself with port blocking (only allow 80/443 into your external firewall).
This takes a LOT of security knowledge and is still a BIG RISK. Another option is to require sharing to be enabled across all users (each user MUST share their root home folder to be compliant, or be disabled as non-compliant).
Keep in mind opening yourself and your users up like this severely violates security best-practice and you had better have a damn good password on your account (eg, KeePass or LastPass randomly generated LONG and random password), not be socially engineered, AND have TOTP (two factor) enabled. Also DO NOT USE the admin account for such purposes.
This kind of function would be similar to auditing functions required by some industries where an auditor must see all files but in a read-only fashion. Do not use the well known admin account for this purpose.
Be aware of how doing any of this is NOT advised from a security standpoint.