Yeah, that’s pretty much it. The “filename” column  (which depending on the list is the Name column if you look at the InternalName property) isn’t something that seems to be available in any way, shape, or form (programatically or otherwise). The Name is the best bet, but it’s not avaialble for the type of selection or calculation you want as only the title  is exposed.

With calculated  columns, there are very few columns you can pick from for your calculation. A funny thing is that you can pick the ID even though it’s not listed. Maybe because it’s universal but you would think other columns would be available but they’re not.

SharePoint isn’t a silver bullet that can solve any problem so with any problem you’re trying to find a solution for, it might not be the right technology.