I understand your use case but I don't think it is really appropriate for this application (outside of a plugin). I could understand the case for the functionality I mentioned in my post but I don't understand why you would use Joplin - which is designed to be a collection of Notebooks and Notes that you add to - to be a simple .md viewer with no intention to add the files to your collection.
A text editor like VSCode or Atom seems far more suited to this. You simply load a folder, which displays the entire folder/file tree and just click through the files you want to read. I believe VSCode allows you to toggle markdown viewing out of the box and Atom has a bunch of plugins that allow you to enable markdown previews. You can run comprehensive searches against all the files within that "project".