PCB design for MAX31865+INA219
You want to keep your switching currents away from your sensitive analog signals. Since the 3.3V is derived from the 12V, dividing the ground plane won't necessarily help. I would have a separate connector for the heater and keep the MOSFET switch away from the analog circuit.
You need to start placing the components to see where the ground currents will flow.
Your sensor is not referenced to ground, so that will help. Read, understand, and follow the recommendations in the ADC datasheet.
Without knowing the application it is hard to know for sure, but if your temperature regulation requirement is not that tight, and your thermal mass is high, this design is not that critical. Software should be able to filter out any noise.