The soaking wet bolt application in the military is usually practiced in training, with blanks and an adapter, which geometrically increased the amount of trapped residue. Better to soak the trainees bolts and let them stay running than have a lot of downtime and complaints "My jammomatic quit!" Stoner didn't attempt to make them run with a .060" bore outlet and the cheapest pop powder that met training ammo specs.
Phosphate is the bare minimum to prevent the BCG from rusting when you are in a severe environment, shotgun it when you're back from a range, lube and wipe it down, it's good enough. Do that every time and no problems. Expensive coatings might pay their way if you took a carbine course every weekend, otherwise, most don't shoot over a 1k rounds a year and normal maintenance would be enough.
What the coated bolt sellers are doing it simply pushing the old "M16's are dirty" myth and trying to play your fears off by getting you to spend money. M16's with issue bolts have been doing well, for decades, in wars, and in storage, there's little point doing more unless you want to dress it up. The coated bolts are on the market because they sell, not because there has been a huge drama over not having it. Just like charging handles - it's really not a big deal.
Shop for milspecs with MPI and testing, you get your money's worth, but if you want a low price, the coatings rarely make the cut. And you don't really need them.