Open-loop Buck LED dimming
There is nothing wrong with running this setup as open-loop. Closed-loop is only really needed if trying to achieve some exact repeatable output, like a voltage regulator. In this case, all you're trying to get is a lighted output and the chances of noticing if the voltage is 19.2V or 19.7 after turning the light on are very small.
Now what I would suggest is to move the MOSFET to the input of the circuit rather than the output. The circuit as it sits will radiate a lot because the switching current is in the entire thing. By moving the MOSFET to the input the switching currents will be filtered before being delivered to the LEDs. Then by placing the buck converter in a metal enclosure, the radiation will be limited. Be sure to provision for a snubber on the MOSFET. Not having a snubber is a quick way to overheat the FET.

Also, make sure that the inductor is capable of delivering the continuous current if the MOSFET is on all the time without overheating and obviously fuse everything.