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?
Could you send it over? Seems good to check it out. I'll be on the lookout for a more powerful transformer in the meantime. I also went out and bought a more reputable SSR still rated at 40 amps.