I had a 2020 Sierra half-ton with the inline 6 (3.0) Duramax. I routinely got around 26mpg when not towing. Of course when towing, got only around 15 or 16 (lightweight cargo trailer/camper). Because it was the stupid carbon fiber bed, I couldn't put a Decked drawer system in it, and no shell would fit due to the contour of the bed rails. After deciding I had to have both the new Decked drawer system AND a shell, I traded it in on a 2024 Sierra half-ton, also with the Duramax 3.0 six cylinder. I don't know the reasons why but GM refreshed the 3.0 Diesel I think in 2023. So now I have the new one. I absolutely love them. They are quiet, strong, very strong, and get better gas mileage than my wife's Subaru. I typically go 75-80 on the interstate when we are heading to dog training or hunting, it carries 4 guys and three dogs comfortably, all the shotguns fit in the Decked system and it rides like a dream. Most riders can't even tell it's shifting with the 10 speed transmission.
I can't speak to durability over the long haul because I haven't had them that long. In terms of additional costs, you have DEF, which I have to add about every 5000 miles unless I'm towing. When you tow, it sucks DEF.
I was surprised but what little additional cost the 3.0 Duramax added to the truck, I think it was only about a $3500 premium, and I think it's well worth it. With a 24 gallon tank, the computer usually tells me I have about a 575 mile range.
Hope this helps.
