PlastX isn't necessarily the correct product for hard plastics like headlights
This is not true, backwards even. Hard plastics do far better with a plastic polish than soft ones do.
and since you don't mention adding a UV coating, it will start deteriorating in a few months. Before and after using a 3M kit + their coating.
UV coating means you must strip the coating off when it degrades and go through that long process again, while not using the coating means you can do a quick, easy touch up instead. Whether it is every few months or a year, or two, depends on where the vehicle is parked relative to sun exposure. A touchup takes only a damp rag and a minute, could be done when you wash the vehicle for example.
You're far better off using a polishing sealant or wax product instead, that you can just polish through with regular plastic polish while it's still on the lens, or if you do it more often, need nothing more than that polishing sealant or wax product.
I'm not suggesting that the kits
don't have their place. If the lens is bad, sure you may need to break out the sandpaper that ONE time, but never again if you keep up with them, unless you put that hard clearcoat on then you put yourself in the position to need a much larger strip down effort and expense again.
It's clever marketing by 3M for sure, to get people to spend more money AND do more work for a result that is often inferior because the additional amount of work tends to make people put off re-doing it longer, especially if pulling the housings to do so.
Lastly, your headlights are shiny but optically more translucent than they could have been. They are supposed to be polished as finely as possible, not left hazy to try to get a clearcoat to adhere better. Adhesion isn't the only issue, rather that even with perfect adhesion, no spray on product stops all UV from degrading the coating and the plastic under it, so you are adding more material that needs removed, so you need a more abrasive solution, so you must go through more steps till the final, finer abrasive again.