So far the theme appears to be Google-proof. I have five of the perimeter entries and have worked out the nature of the thematic items from the hint (and some reverse engineering), but no combination of these, along with "writer died 2021" etc, produces any useful search results. The added letters don't seem to make any sense either although I haven't quite finished the grid fill so I'm hoping enlightenment will come, but that will have to wait until tomorrow. I'm way late for dinner!
So far this is a slog, but the puzzle's brilliance may become apparent on completion...