Ahhh, the pleasures of getting a tank setup right...

Here are a couple of suggestions for each issue:

Humidity: Using foil is a cheap and good way to cover a screen top. It also won't be affected by a heat lamp. I would say however, don't cover the screen top completely.

Make sure to leave some portion open for air exchange.Perhaps you're already doing something such as misting sphagnum moss in the tank. This will definitely help, but you may end up needing to mist it every day (annoying). Something I do with my tank is to take pumice (a rock) and soak it in water in my sink. Then I put the stone in my enclosure. The porous nature of the stone allows it to retain a lot of moisture and let it out gradually. A few good sized pumice stones might help keep the humidity up.

Temperature: Good call on using a UTH, they also won't dry out the enclosure as much as a heat lamp. Question about your temperature probe; is it just to monitor temperature or does it control the UTH? You could get a temp controller (Inkbird is what I use, $30 on Amazon) that has the UTH plugged into it. I set mine to 93 during the day and 83 at night. This way, you're snake will always have at least one sufficiently warm area. My ambient air temp varies between 73-76, which is fine. I don't think that a ceramic heater is necessary, especially since they are notorious for drying out the air (which is an issue you're having). I use a 50w infra red heat lamp atop my tank on the hot side, which rests atop the screen. I use a timer for it where it runs from about 10am-midnight. It isn't too powerful, but I have sphagnum moss at the bottom of the tank below the light, which is misted and allows humidity to be generated. For a tank you're size, a 75w infra red heat lamp might be good.

Hope this helps!