Per user encryption
There are three ways of encryption: https://help.nextcloud.com/t/per-user-encryption/42556/3
All variants have some drawbacks in usability.
Share owners may need to re-share files after encryption is enabled; users trying to access the share will see a message advising them to ask the share owner to re-share the file with them.
After all, users need to trust the sysadmins of their hoster ;-)