Uh... You described next cloud. Not sure what exactly you dislike about it.
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A place to share alternatives to popular online services that can be self-hosted without giving up privacy or locking you into a service you don't control.
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I have no idea what you want to achieve.
Seafile has SeaDrive which mounts like an external drive and works just like other similar services like OneDrive.
Synology.
Have to buy the hardware, but 1,000% the best Google Drive alternative and 100% self hosted.
Two ideas: First, you're describing exactly a traditional SFTP share with the same files being in a location but each user can have different permission levels. Also, samba shares does the same thing. Not sure if a modern web UI is a requirement for you though.
Second, you could use nextcloud and just share the files, or use groups (or circles I can't remember what they call it).