two additional strategies that might help:
1 - bring them to the net by hitting a drop shot, a short angled ball low ball or approach off of slice. this will make their shots deeper in your court.
2 - mix up your shots in terms of topspin, flat and slice shots and also mix up your placement. You're too predictable so they can slice you easily, if they have to adjust their stroke or run to a ball it makes it more difficult for them to slice and their ball might float a bit more. You will have to be able to deal with coming in to finish points rather than sitting back and grinding from the baseline.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM