Best tool for removing small area of thinset?
For a small job like this I would use a hammer and a hand chisel like this Make sure to use safety glasses it is wild how far some of the chips can fly. I would inspect the Joist's on both sides of the toilet if the area has rot these may need to be repaired and part of the cause for the sinking. If you would like a new hammer drill most 1/2" models will work quite well in this application without the heavy impact of a hammer and chisel (home models).