I saw this new product from Meguiar's on TV the other day:

Perfect Clarity Headlight Restoration Kit | Meguiar's

It includes a protectant spray that "Lasts up to a Year".

FWIW and YMMV

BTW-I've done dozens of headlights over the past few years using your time-tested method: sand from 800 thru 3000 and polish. The "layer" you speak of is just that. Either a sprayed-on or a laid-in layer of a tough polycarbonate before the Lexan (a different polycarbonate) is injected into the mold.
Tough stuff, 'eh? Once that layer is damaged, it must be removed. I've never found a way to polish/repair it.
Of course, once it's gone, the regular structural Lexan is exposed to the elements-and UV. Either polish it out every 3-6 months or re-coat it with something like this stuff.

Or XPEL/ Lamin-x films.

And that's all I've got to say about that...