Ok, clearer now. What you need is a low power (500-600W is about as low as they go) mains powered pump with a pressure control box. Your solar is more than adequate for that.
You need the electronic control rather than the mechanical one as the boiler temperature is dependent on the water pressure. The pumps with mechanical pressure switches and an expansion vessel do not give a constant pressure so it's difficult/impossible to get a decent shower.


As you've discovered the 12V pumps don't work..