It’s possible the flicker is being caused by running at too low of a frame rate, add setDither(0) to the end of the addLeds line to turn it off:
FastLED.addLeds<CHIPSET, DATA_PIN, CLOCK_PIN, COLOR_ORDER>(leds, NUM_LEDS).setCorrection( TypicalLEDStrip ).setDither(0);
(At some point mark and i will add something that adjusts the dithering on the fly based on the frame rate you’re pushing out updates at, but we’re not at that point, yet).