What an intriguing post. I am not sure where to start, but I like what you are trying to do.
First, I am a barrel and scope nerd. Strike that. A barrel nerd and a scope snob.
First point of order: do not worry about the FSB. Once you get aiming beyond 80 yards, it will all but disappear. It is not going to get in your way. If you think it will, then get a rifle without the front sight post. You are not going to use it much anyway if you get a LPVO type scope at 2.5-10x, let's say.
Next, the rifle begins with the barrel. If you are looking for 20", you are looking for a relatively accurate barrel, so yes, the chrome lined barrels are probably out. I would not discount them totally, but you are going to want -- from what I see you write -- a match barrel, so not military grade chrome lined. From there, I would say you do not need to spend a fortune. On the best of days, you will be hitting 1/2 to 3/4 MOA, and most will claim 1/2, but are usually hitting 3/4 to 1-1/4 MOA. The question is not which is the best barrel, but in today's supply chain, who has a barrel available for you in match grade, or if you want one made, how long will you wait and what priority will you get. You might. I would put Criterion, Lilja, and Douglas blanks to the front of the list. You can pay more for Bartlein or Kreiger, but probably will not see the difference in a 20" 5.56 gas gun. And, you can also look at 18" barrels, which might be more available. That is up to you.
Keep in mind some of the names you mentioned are not barrel makers, but barrel spinners, or barrel smiths. Craddock uses Bartlein or Criterion barrels, for example. Both are imporant, and there are a handful of really good barrel smiths, but they are also very backed-up with custom work, and I do not think you need custom work. If you think you do, then go for it. It is your money, and White Oak or Craddock or Compass Lake will custom-make whatever barrel you want, but they might deliver in May.
Then, build your rifle around the barrel. A solid receiver set, a BCG that will not fail and a good trigger. If you are going for FSB, I would still favor the military look and function of forged.
For scopes, we usually look at 1.5" as the minimum height for a 56mm 0MOA rail and mount cant, so you can go lower, for sure with a 44mm objective lens, but the front sight base will cause you to want to higher. "cowitness" is in the 1.4" to 1.5" range, and what I prefer with a front sight base is 1/3 lower co-witness actually puts you a little higher at 1.6" to 1.7"
Now for the scope. I think you picked a solid starting place with an NXS, and 2.5-10x is a good magnification for the range you will be shooting. I am not even going to confuse you with other options. That is a good one.
Hope you enjoy the endevor.
