1) Why are you driving the high side of the bulb instead of the ground side.
Ground side is much easier becaust eh source is at ground so the gate driver
can be referenced to ground. Unless it is a P-channel and VIN is less than
15V you're probabbly better off with a low side N-channel (they are more
robust).

2) Why use Comverter.  Average should be fine as you are only going to
create a DC average.  for average its as simple as a R-C if the pwm
frequency is high enough or a multi pole active filter. if it is lower.

If you comare average voltage in with a variable reference, you can have a
adjustable brightness curcuit that is linar with your pot adjustment
andfixed from external changes in VIN or temperature.