The problem with rheostats vs thermostats is that rheostats lack a probe to actually measure the temperature. As a result, the setting that works perfectly in the winter, burns your snake in the summer (or even A/C on vs A/C off). The rheostat just controls how much electricity is going through(in broad generic non engineer terms). A thermostat turns it on and off based on checking the temperature.

If you have stable house temperatures, a rheostat can probably work just fine, but you have to make sure you monitor the temperature yourself(temp guns are great for that. Get one if you don't have one already).

As for the humidity issue, as a tank owner, I struggled with that for a long time. What I eventually did was get a piece of acrylic plastic from home depot and cut it so that it covers about 2/3 of the lid. I also put the light on a timer so that it wasn't on all the time. Now my humidity is regularly around 70% without misting and can go higher as needed(in shed for example). Aluminum foil helped in the past, but I found it tends to curl up at the edges and let the air(and humidity) out a lot more.