You can not take away any copper that you don't need for heating, that'll help a lot, and if you can connect those to a safe ground trace or plane: First of all that will conduct the heat away to that plane, which helps, but it also avoids resonances and such when you start PWM-ing the heater.

You can shield electrostatically with a ground plane, but the magnetic field will go right through everything, so if you have sensitive circuits you might need to filter the PWM to something more like DC.

I'd use a cheap SMT thermistor for the sensor assuming you only want to control at one temperature. Just stick it in and it will work.