When you picked out the cheapy thermometers, did you pluck 10 off the shelf and compare them all and see what ones were reading the same and what ones were a bit higher or a bit lower?
I learnt that trick with hygrometers for my snakes. There are always some screwed from the factory, so pick a few and compare before purchase.
I cant make any imput on the controller. My tanks old school, i only use one of those cheapy glass thermometers
i check what it reads in the tank, then pull that same thermometer out and use it in my buckets when i make new water so i know 100% my new water is the exact same and im not relying on different equipment.