If you want to go beyond cork in your polishing, cerium oxide will give an optical polish.
You need to grind your glass at 800 or higher grit, or use the cork belt after 400grit. Any rougher surface will not give a smooth polished surface. It will only polish the high spots.
Many do not like to use cerium oxide as it is messy. Especially so on a wet belt sander as the speed is really too fast for the use of polishing pastes. The speed sprays the slurry all over the place.
You need a felt wheel or belt to which you apply the cerium oxide. First you mix the cerium with water to a yoghurt consistency and apply that to the wheel or belt. Begin polishing and add more water and cerium paste as the polishing surface dries. You will notice this as the glass will begin to drag. Do not delay, add more of the paste before continuing. Otherwise you will heat up the glass and risk breakage.
It is helpful to mark the glass with a paint or china marker before starting the polishing process to show the areas that are to be polished. This enables you to see what work has been done without completely drying the piece.
For large surfaces you will need to use a horizontal grinder with a polishing pad attached, or a hand held polisher.
An alternative is to use “trizact” belts that are about 4000 grit. These achieve a polish that is very good, if not as optical as with cerium oxide.