You should never have to go lower than 320 grit. When you see your sanding disc is not removing material anymore, you need to change it. If you keep sanding with the same disc you won't get much done since the abrasives in it are no longer working.
You did a great job on the headlights, however I still don't understand why people keep insisting on putting furniture varnish on headlights with products such as Opti-Coat on the market which is not expensive and easier to apply, not to mention it should last quite a while longer.