This might make the whole build more complicated than it's worth but to avoid the bubbles around the heater to an effect on the resin you could construct a double walled containter where the outerwall has water with the heater and the inner tank is the build area.
You could even add some stirring or or similar effect in the heated water to improve heat transfer.

Another option to improve the heating and keeping the temperature steady to have the water reservoir heated aswell