You won't find a 2.500" buffer that you can get to 5.4oz, ever. The max you can get that thing is 5.3oz, with two tungsten weights, inside a stainless steel body. But that works. The Sprinco Orange spring was specifically designed for a 2.500" 5.4oz buffer, running inside a receiver extension that's only 7.000" internal depth. They made that spring, for that recoil system, for a .308AR.
You can run that setup, and it'll work. I run it in my .260 Rem AR, just to prove that it works, and that gun RUNS... I did that for THIS reason. That's the ONLY .308AR that I have with that recoil system. Every other one runs the Armalite AR-10 Carbine Recoil System. My .260 is the one, though. It's the goods.
So, next for you, is verifying your gas port diameter. Your gas tube position is perfect. Verify your gas port diameter. And fix your recoil system. You do that, and make sure your gas port diameter is right, and your gun will run anything you throw at it.