For a 20 gallon tank a 50 watt bulb is fine if the temps you listed are constant. A 75 watt bulb is going to create a hotspot that is to warm in such a small tank plus like you know it will kill the humidity. I say if its not broke don't fix it.

On my cool side of a 40 gallon tank I have a 60 watt bulb for added heat cause I keep a cold house. Ihave the fixture on a lamp stand and have it raised a few inches from the screen because it was creating a hotspot that was to warm. A 75 over a 20 gallon would certainly need a dimmer to bring it into a proper range or a lamp stand to create distance to drop the heat reaching the tank floor.