You have a combination of problems...a system needs to be engineered if you want it to work well together. I'm not sure this will. The pressure variations make it tough, along with the tankless varying its outlet temperature based on the flow rate which changes with the pump cycling. You could try to change the shower valve to a thermostatically controlled one. I was in a hotel in London with one, the flow hiccupped, but the temperature seemed to be pretty consistent. You need to bug the manufacturers to find out the reaction time for sensing temperature variations...not all are created equal and none may do what you want. What may work out best is a small tank fed by the tankless. Then, it would act like a buffer and give more consistent temperatures.