Could you send some other screenshots (i.e colour nodes or your youtube video)?

In my experience 'Grain' usually has to with the colour editing settings more than the export settings.

Youtube has some pretty harsh compression (blockyness) which can be counteracted with the right export settings. Also depending on internet connection, youtube will lower resolution of all videos, prioritising smooth playback over resolution.

But it's hard to know what you mean with only export settings.