Insulating the back and sides of any tank takes care of at least 60% of the heating problems...usually. Also, insulating the screen with the foil treatment helps with most of the remaining temp problems and stabilizes humidity...again, usually. There will always be those who try everything and still just can't get the tank to work for them. I'm just saying it IS possible to get a tank to work beautifully.

21" sounds a bit high...or are you stating lxhxw? With 21" of height, you'll have a harder time trying to get the heat to stay near the floor where it belongs. Under tank heating is preferred, so it's good that you have a pad already. it should cover 1/4-1/3 of the tank floor to be effective.
Usually, to keep temps at the right levels all you need is the UTH and a controlling device, preferably a thermostat. Lamps are only really good for boosting ambient temps and viewing, since they suck the humidity out and the heat from them rarely reaches the floor, especially in a tall tank.

If the pad you have doesn't give you high enough temps, it could be a pad meant for small animals (that only get up to 85), it could be defective, or you could have too much substrate in the tank. Test any pad by plugging it right into the wall with NO snake in the tank and use the probe of the thermometer right on the pad to monitor the rising temps. Most pads should reach 120 easily. This is why you need a t-stat.

Run your tank with everything in it but NO snake for a few days to see where temps and humidity settle, then go from there adding or subtracting until you get a good steady range.