Great post, thanks!
Recently did mine with the 3M kit, but couldn't find anything definitive on what to protect them with, other than the uber pricey German 2K product, which is not available locally. So, I decided to try Rust-Oleum's American Accents Clear (which does not declare if it is acrylic, enamel, or other on the label....). Almost immediately, it started to crackle, looking like a piece of alligator hide! Went back to square one, had to use 80 grit and a palm sander to get off the finish, and start the polishing process over again. This time, before spraying the clear, I prepped the surface with Dupli-Color (Auto Zone) Adhesion Promoter, 2 coats. I followed with 3 coats of Rust-Oleum, and it made a huge difference. No crackling or spidering! Clearly, the plastic lens surface needed to be prepped to accept the clear coat. https://photos.app.goo.gl/upvGfvWPHKzg3O6B2

I bought Meguiar's Headlight Protectant for mine after doing them but I haven't used it yet. I plan on replacing mine with refurbished ones this Christmas so I'm waiting.

I'm coming over from the E38 7 Series, and know that you can just purchase new lenses for the headlights...

Is there an option for the E53? Or are the headlights one piece?

TIA!

No - the lens is glued onto the housing as a single piece

Depends on whether you X is prefacelift or not. The prefacelift lenses can be removed and replaced. Post-facelift cannot. At least not without cutting them off the housing.

The changeover from the glued lens and 'permanent' lens was was not associated with pre or post facelift X5s. The change was made, if my memory serves me right which is very questionable was June of 2002. I have removed a 'permanent' lens. I would never do it again, took me forever to do without damaging the lens. Headlights have a date on them. There are many threads on headlight restoration. If you do a search you can confirm the changeover date.