MO, it's best to give the snake eating what it's used to a couple times before trying to switch it over to f/t. Your snake won't starve if you give it a week or two to settle in.
You have two UTHs but only one thermostat? You know that an unregulated UTH can get hot enough to burn your snake? You need to unplug it until you can get another thermostat.
I would also swap out that analogue thermometer for a digital one, they aren't the most accurate and your snake could pull them down and get the adhesive stuck to them.