I recently discovered that the Files app on iOS/iPadOS can connect to SMB servers, so my solution is to just use the NAS that I've already got. Windows, MacOS, and Linux can also connect to SMB just fine and I'd be shocked if Android couldn't also do this.

Rule 1: There are user accounts, but they're local to the NAS. I think it meets this one.

Rule 2: Setting up the NAS is likely out of reach for many non-technical users, but using the files app and doing stuff in it is perfectly reasonable. Partial marks here.

Rule 3: It's consistent in the sense that every platform uses its native file manager to connect to the NAS and manage files. Managing files on each platform is different, but you're already dealing with that difference. I'm calling it a pass.

Rule 4: The NAS is the junk in the middle, so this is a fail.