0.5-1.7 mm sand is what is recommended for sand sifters. You could probably safely go up to 2.0 mm. Black Hawaiian sand has some grains that range in size up to 3.5mm, which are the larger pieces mostly on the top of your substrate. The larger grain sand usually ends up settling on top of your substrate.
I still believe it is most likely caused by gill damage, based on what I see. (Their gills can heal, btw) It doesn’t appear to be eating either in your video. If you can take some closer still photos of the goby it may help too. If it is only doing it strictly at feeding times, though, maybe he just is doing a happy dance before eating and you don’t actually have anything to worry about? Keep us updated and you’re doing a good job monitoring the situation it sounds like!