Update.
I read about heat traps and concocted something like this:
The heat trap goes up ~35cm, then down ~20cm, through the pump and up to the radiators. The heat trap is
above the mixing valve. Could this be the reason why it doesn't work? The heat still siphons up through the pump and to the radiators
Here is a simple diagram:
During the summer I plan to rework this so that the heat trap is before the mixing valve. Since the low ceiling limits the height if the heat trap goes up above the tank, then can it go down, like this?
If this will work, how low should it go?