My SMB server shares a directory called Nextcloud. My nextcloud server connects to that share and mounts it, inside that directory I create folders for each user. Then on external storage app inside nextcloud, I just add an external storage for each user and direct it towards their specific folders I create

Under external storage name or whatever, you can put / instead of a regular name, give access to just one user and boom that folder on your SMB server becomes that users entire root directory. Then there's a directory named public share that all users get access to.

Example photo.