What's so tiring about them?
They work pretty well, lots of surface area, you can't put much flow through them though so they don't really work that great for anything but a small or low load tank. The major advantage for breeders is they don't suck up fry. So running a sponge filter+an internal would defeat the purpose of them. Putting a sponge on the intake of an internal filter would work or running them with a powerhead. But then they clog up very quickly, not a problem if you clean them daily.
They work well with severums. Mine bullied the severums. I could swear I saw a confused look on the severum's face when this 1" fish would try chasing away an 8" fish that probably outweighs him by about 800 times.
Severums definitely. Much MUCH hardier fish. Discus are like the poodle being carried around in a handbag of the cichlid world.
Probably yes, possibly no. If they're full grown they're less likely to be eaten, but in general if it fits in another fish's mouth then it's food.
Ok, so you don't actually need an aquarium heater then, from the sound of it.
But all stainless steel heaters have seperate thermostats(I think) and they're mostly around $50.
There are a handful of heaters with remote thermostats. You could cut that thermostat off and connect it to your own. Otherwise you'd need to turn the heater's thermostat up higher than the setpoint you want the water at, but few will go as high as 36.