Thanks wvmarle and GoForSmoke,
wvmarle indeed heat distribution might be a problem that i didn't consider. I will make a setup anyway so then I can analyse how severe this problem is. Keeping it liquid all the time would require a lot more energy since it may take a month to have a full tank and then someone will come and empty the tank...
So it could be melted just one day beforehand which reduces elektricity use of the heater with a factor 30.
GoForSmoke, I was also considering this at first. Afterwards a was concerned about overshoot but now I might reconsider and try this in my setup, thanks! The MCU is used because this is only a part of a bigger project (sigfox, LCD, level tank) but I only explained what is relevant to my worries/problem.