A properly maintained Colt AR-15 should eat in-spec 556 steel or brass ammo like candy.
For $4500 (or less), you can get a SCAR-17S off of GunBroker.
What is the exact make and model of the 7.62 AR and what optics come with it? Is it new or used?
Can you afford to shoot both your 556 carbine and your 762 rifle? If not, I'd focus on whichever you can get ammo, spare parts, and mags for. How much better equipped would you be with a 762 semi-auto versus a decent 762 bolt gun with top of the line optics?
Given the current ammo shortage, any 556 that reliably cycles in your AR and is within your budget is what I would go with. If that's Tula for you, then there's nothing wrong with that. If you want brass, be prepared to pay silly prices for it.