SET_MAX_POWER_IN_VOLTS_AND_MILLIAMPS(5, 500);               // FASTLED POWER MANAGEMENT SET AT 5V, 500MA.

TO CAP THE POWER USAGE AT 500MA.

BUT THEN, IN YOUR LOOP YOU HAVE:

FASTLED.SETBRIGHTNESS( BRIGHT );
FASTLED.SHOW();
///// BPM CODE END ////
SHOW_AT_MAX_BRIGHTNESS_FOR_POWER();                         // POWER MANAGED DISPLAY

YOU ARE CALLING BOTH FASTLED.SHOW() AND SHOW_AT_MAX_BRIGHTNESS_FOR_POWER.  I CAN GUARUNTEE YOU THAT SHOW_AT_MAX_BRIGHTNESS_FOR_POWER IS GOING TO PULL THE BRIGHTNESS DOWN PRETTY FAR FOR 300 LEDS WITH A 500MA CAP.

SO, AS YOUR BRIGHT VALUE GOES UP - YOU WILL HAVE A CALL TO SHOW WHERE THE BRIGHTNESS WILL BE HIGH, SAY, 200, OR 240, OR SUCH.  THEN IMMEDIATELY AFTER THAT YOU HAVE A SHOW_AT_MAX_BRIGHTNESS_FOR_POWER WHICH WILL BE A LOT LOWER - AND THERE'S YOUR FLICKERING.