Let's start with the elephant in the room: I have no formal education and a few small servers that I have to manage outside of the topic for a variety of reasons. This means that my knowledge is sketchy. Please wear with me.
One of the servers is reserved for a few users (less than 10) and I was asked if I can make an accurate decision about the file access policies between the users.
By that I mean that my users want to be able to grant access to their files per file and per user. For example, User1, User2, and User3 can access FolderA, while User2 and User4 can access FolderB and FolderC only for the owner.
They more or less want Dropbox-like access management management for their personal files, just for their local files, between local users.
I could not find anything suitable, so I'm wondering if this is a feature that is meaningful to activate in terms of security and feasibility. I admit, I've never thought about it, but it's a feature that makes sense in a multi-user server for cooperative tasks.
Since groups are not a solution (I need up to 30), Nextcloud is too much of a good thing and not for active projects you're programming for. Maybe git or PAM?