Loss of quality while adding subtitle via FFmpeg
As you are hardcoding subtitles, the video (with the subtitles added) will be re-encoded. 

You can use the CRF rate control method to modulate the quality of the output.

So, start with

ffmpeg -i grdedFinal.mov -vf subtitles=portSbs.srt -crf 18 -c:a copy gradedFinalwithSubs.mov


If the quality's not acceptable, lower that value till it is - in exchange for a larger file. Of course, don't decide based on file size but on visual quality. x264 is very good at compressing video while maintaining subjectively perceived quality.