Is it a good indicator that you hit the upper end of pressure when you don't see any carbon on the case neck? I have no heavy bolt lift, extractor marks, primers are rounded, and easy to extract.
That is not how I would put it.
First, unless you get into one of those odd areas of too little powder created a pressure pulse, then carbon on a case is too little pressure to seal it.
I get leary of gas escape aspects and its a gun crud up as well as brass clean up, I move them up.
I have never seen carbon when I got the pressure too high.
As you correctly noted, hard bolt lift, sticky extraction and there are no ejector wipe marks are saying you are fine. You can assess if you go higher based on what your load level is vs the books and how close you are.
Primers are low on my list, its not that I don't look at them but its a primer compared to a lower load of the same powder and the other factors that are possibly relevant.
Simply put, if I have any of the bolt stuff I stop and go down. A primer alone, no. Too many causes to be a "here be dragons"
I look at it as going into a mine field. Ok, someone marked it (developed loads and came up with a max) and I am going to be carefull.
But, if you are all good and your load is doing what you want (accuracy be it bench rest or hunting good) then you are in calm waters.
My take is I go with what my barrels tell me. If I could get under 1/2 MOA and I was shooting a ridiculously low 1500 fps with the 06 (however I got there) it would be, cool, I am good with this.
I marvel at the people who want 2800 FPS out of an 06 (175 gr area) for what? Hard on you, hard on the barrel and the drop at the further ranges is not changed to any real degree (at 600 yards its still going to be a lot so a 1/2 inch better is?)