Looks like your UTH is an RH-5, rated for 30-40 gl tanks. I use one(1) RH-4 on the warm side, with a 10 gl tank. I still have to keep the rheostat (Leviton) turned way down for optimum temps.

Also, those bumpers on your UTH should be installed on the tank base. You need some airflow between your tank and stand. I tossed the bumpers and used plastic leg caps for washers/dryers. It allows for about a 3/8” clearance.

And as others have suggested, I am moving on to a good thermostat, as well.