Hello Issac,
Instead of using Set Brightness I recommend to work with Raster Calculator dialog and use False Colors representation mode.
You may need to use some normalization coefficient to get the values of outbut bands within 0-1 range, for example: B1 / 32768, B2 / 32768, B3 / 32768.
Then in the orthomosaic export dialog select Index Color option to save the false colors representation to KMZ file.