Uh... You described next cloud. Not sure what exactly you dislike about it.
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I have no idea what you want to achieve.
Synology.
Have to buy the hardware, but 1,000% the best Google Drive alternative and 100% self hosted.
Seafile has SeaDrive which mounts like an external drive and works just like other similar services like OneDrive.
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).