Hmm, the heater element being controller disregarding your will is a bit of a bummer.
Thanks for clarifying, so you are indeed circulating the water itself, it’s not a separate line used as a heater. In this case you really are at the mercy of either your pump, pipe length, ambient temperature, insulation and/or sun intensity.
Have you monitored the temperature at the panel on the roof itself? Also, what type of insulation are you running on those pipes?