Single tile and drywall repair with a backing stud
I would cut out that square.   Cut drywall to size and get 3-4 screws on that stud.   Little thinset... might even use mastic here.    Pop another tile over it (need to cut hole in tile and take the inlet valve off - TURN OFF WATER).   There is no reason you should overdo the wall behind a toilet.   No one will be pushing right at the edge of that tile... and if there is someone doing that they are the problem not your fix.