Also, one big problem you will encounter is this:
If you lowpass filter the PWM signal to get an analog control voltage, and then apply that to the LEDs, you will get illumination that is exponentially related to the voltage. Your illumination vs voltage will look like the current vs voltage curve of the LED. Is that what you want? I doubt it.
If you use a voltage to current converter, you will get linear response, but you will never reach the full 3.3V across the LEDs.
If you lowpass filter the PWM signal to get an analog control voltage, and then apply that to the LEDs, you will get illumination that is exponentially related to the voltage. Your illumination vs voltage will look like the current vs voltage curve of the LED. Is that what you want? I doubt it.
If you use a voltage to current converter, you will get linear response, but you will never reach the full 3.3V across the LEDs.