I'm looking for a way to mark which files I have viewed/read and the ones I haven't.
marking the most important files for future reference would be very useful.
There is no function to mark files.
You can though create a new folder in your drive and create a shortcut in it by right-clicking the files you have viewed/read.
In Google Drive you can
Create workspaces and add files to them
Create shortcut to files
Nowadays, Google Drive has quick access to Calendar, Keep and Tasks. They could be used to create events, notes and task with links to the files.
In combination with your web browser, you can bookmark the files.
In combination with your operative ...