The controller is working as intended for you. On default settings the containment field stabilizes around 20%. I WILL make a youtube video because many ppl have trouble setting it up correctly it seems. Anyways - your setup seems to work fine.
Do NOT worry about the temperature as long as it is not above 9000. The default settings are going to throttle the reactors output between 8000 -> 9000 C (= it's adjusting the target saturation from 50% to 90%) between 9000 and 9900 the output rate will be reduced from 100% to 0%. Anything above 9900 will trigger an immediate shutdown (although