As a guy who cuts granite every day including the very hard Egyptian granite probably under discussion you need only be absolutely sure of a few angles/lines and you can take it from there. Also, I’d very much doubt that the carvers of antiquity could out perform present day high speed computer guided blades.
Could you accomplish what you do today with copper chisels and round pounding stones, because those are the tools we are told the Egyptians used.
What’s interesting is they were able to do things that just aren’t possible without more modern tools, that Egyptologists claim weren’t available to them. The problem is this would upend a lot of the history. So they just kind of ignore it. This in turn, allows for a lot of wacky speculation, because of the vacuum on the subject.
A guy named Flinders Petrie documented a lot of his finds. He discovered some granite core samples made with tube drills and noticed some interesting characteristics based on the cuts observed, he estimated a feed rate of 1 in 60 or about .100” cut into granite per revolution of the tube, which he found astonishing and probably not possible at that time (1890s)
What’s weird about the big granite boxes is they are made from a single piece, often 20 or 30 tons, 90 degree precision internal cuts and polished and then transported from hundreds of miles away. Nobody does this today, even if they could, it’s always done in separate pieces.