Good job on planning!
I only see 1 thermostat listed, but, three heat sources: UTH, red bulb, white bulb.
Note: I'm not a big fan of heat bulbs since they don't work well with some thermostats and they always seem to burn out quickly on me. Since a 20 gallon is not very large...instead of bulbs I would use one ceramic heat emitter (150W) with an appropriately rated reflector housing. Covering the top of the screen lid opposite the heat emitter will allow you to retain more heat/humidity. Insulating the sides/back of the 20 gallon with foam insulation board would also help stabilize temps. if you live in an area with cold nights/winters.
Another handy tool not on the list is a contact surface temp. gun which would allow you to ensure that your UTH is not overheating and the surfaces in your tank are not too cold.