Fahrenheit is also based on water’s phase changes,

Exactly: It puts 180 degrees between boiling and frozen, as though they were opposite conditions or something.

It’s set at 32 instead of 0 because 0 is the temperature of the most stable frigorific mixture they knew of. If you don’t have access to a reliable thermometer for your lab, the thermometer you make and calibrate with Fahrenheit’s brine method is going to be more accurate than the one you make for Celsius’s freezing/boiling method.