Multiple wordpress installations that shares Wordpress & Theme but with separated database

I'm searching for a solution to a architectural problem with an Wordpress application I've made.

Each instance of the application is running on it's own subdomain (a folder on root) and right now, each of these instances have a complete copy of Wordpress, the theme and the plugins, and of cause their own database.

My dream scenario is, that each subdomain instance share Wordpress, the theme files and plugins. The uploads and database (namely posts and users) are not shared and must never be "mixed" so a user on subdomain instance #1 cannot log in or access anything on instance #2.

I've investigated multisites but that's not the perfect solution in that the multisite is designed to share users across network and I really need to make sure security and seperation between users on different subdomains.

Another solution, which is more a better-than-nothing solution is to define the content folders for each subdomain to be the same in root (WP_CONTENT_DIR WP_CONTENT_URL) and then defined uploads to be within each subdomain folder. This make it easy to upgrade the theme and plugins but I still have the hurdle of updating all subdomains Wordpress installations. But as said, better than nothing.

Yet another way to go that I've examined is multi tenancy for Wordpress. But unfortunately I've not access to SSH or to the system (Unix) or webserver (nginx) and I cannot get access (it's on a web-host) and as far as I can tell this is needed to make a multi tenancy installation to share (perhaps via symbolic link) Wordpress and content.

Is there any other way to accomplish this structure or is my best chance to kick out my host and just buy a server (space) and manage the installation myself, including SSH and make it multi-tenant?