am regularly amazed that we pretend folders are the right way to organise files. They’re entirely arbitrary. Every competent file system ignores them to its best ability. Why can’t I have a file in two folders? Why does one have to be a “reference”? Why can’t I filter for files that exist in 3 folders with X extension?
We’ve been played for absolute fools.
No, in nearly every case, you never want a hard link. You want one file, and symlinks to it. (Technically every file is a hard link to an inode, and subsequent ones are just additional links to the same inode.) In ext4, you can’t easily get a list of links to an inode, you have to scan the filesystem and look for duplicates. Other filesystems might make this easier.
You shouldn’t try to use a tree filesystem to approximate a tagged database. Use the appropriate tool for the job.