You stated that the AR-15 receiver & bolt are good. Have you priced out the features you are thinking about replacing?...if so, have you compared those to the cost of a new rifle which would, in your opinion, would be more acceptable than the one you have now? Should you decide to keep your current rifle, I would suggest that you start the parts replacement on a step-by-step basis to observe if that item is making an acceptable difference, and if it were me that first replacement would be the trigger assembly, because that is one of the key factors for improved groups down range (others, not all, are your shooting technique and shooting experience); the next would be the optics; and lastly (if necessary) the barrel…I put the barrel at the end of the list, because, unless you are looking for an integral muzzle attachment, or to change barrel twist, and/or to do some serious target shooting, the barrels on AR-15’s these days are usually good enough to provide acceptable performance for the reasons you listed above; if you find that is not so then replace it. Good luck.