The clock is just sending pulses every IVAL seconds, so there’s really no reason to RST it. Sounds like you want clock to be enabled only when delay is not active. You can do that using delay-ACT > “not” > defer > clock-EN.
I’m not sure you really want delay activated by delay-ACK. That will also cause delay after heater turns on as well as off. You could link “less” > pulse-on-false > delay-SET instead of using relay-ACK.
A “safer” option (if you don’t want things getting too cold) is to turn off above 50, but wait until something lower (like 45) to turn back on.
If it still pulses on/off too rapidly, your temperature probe is too close to the heater or you are measuring the wrong place (for example, if you are circulating a small amount of water to heat from a large tank, don’t measure the small volume just after the heater; as soon as the heater turns off, you are measuring cold water from the tank again; measure before the heater or from the tank).