NiB is brittle and any imperfections in the coating will eventually result in peeling or complete catastrophic failure. My opinion so your milage may vary but M. Phosphate is good and chrome is great. DLC is fine too but expensive for what it is. People love to overthink this, it's easy to do.
If we're talking backups to provide replacement parts for one rifle in desperate times.. at least one new complete bolt would be good to have. As well as a replacement extractor, extractor spring, a good amount of gas rings, an extra firing pin, spare FCG springs, replacement buffer pad and rollpin, and a couple barrels. Magazines with good springs are a must as well. Action springs can wear out. Tubbs flatware magazine and action springs will last over your lifetime.
Having backup rifles is also not a bad idea so you have a ready replacement, will allow you to repair breakages later and not die as your trying to replace the bolt in your carrier.