My Candy Apple is too brittle, how to prevent?
As already suggested, maybe you should try for Soft Crack instead of hard crack.
If you are unable to rely on the thermometer just try a cold water test. Drop some hot candy in to iced water. Stop cooking after the hard ball stage. As soon as it will form pliable threads.
See

https://www.instructables.com/id/Candy-Making-without-a-Thermometer-Cold-Water-Tes/
https://www.thespruceeats.com/making-candy-without-a-candy-thermometer-520309

These tests show what happens as the candy cools, and so predict what the final candy will be like. Even if your mix needs a different temperature from the 302 F you are aiming for it should work.
Make a note of what the indicated temperature is on your thermometer, and if the candy apples are ok then on future batches just use that thermometer and that temperature.