Wow. A lot of questions. We had some of the same struggles with our 20L, too. Here is how we solved it:

Cool side has a UTH under it - with cardboard and shredded Aspen to make sure the glass didn't get too hot - this ensures the cool side doesn't get too cool. The UTH is on a dimmer (rheostats are more expensive, a dimmer is cheaper if anyone over there knows how to do basic electrical work). Cool side also has vapor barrier paper (for under wood flooring, but you can use something else) with one hole in it for ventilation.

Warm side has a CHE (ceramic heat emitter) of 100 Watts in a Fluke's wide brimmed lamp, sitting on to of the tank (bulb is far enough up not to heat the screen too hot). It is plugged into a thermostat that turns the CHE up/down/off/on to keep the warm side temp between 89 and 91 degrees F. The lamp sits in the large whole on the vapor barrier on the warm side.

We have digital thermomaters and hydrogometers on both sides.

The tank is insulated on the back and right sides with 1" foam.

The tank sits next to an elephant humidifier (holds only 1 gallon). We have it aiming out at the room, but it keeps the tank between 48% and 57% humid. Now that our BP is close to shed, I aimed the humidifier toward the tank to bump the humidity 5% to 10%.

Be careful with misting too much - wet does not equal humid. And I think that is how we gave our BP an URI. She is done with her antibiotics now.

Good luck!