It seems that a large dealer, or one that at least sells a cosiderable volume of these SUVs, would be where you want to order if you would like one sooner than later. I ordered a 2024 Yukon XL Denali with the 3.0L from a small dealer (Dealer 1) in October, who couldn't tell me if/when they would have an allocation, so then I ordered a second identical vehicle through a larger dealer (Dealer 2) in November that I know sold a fair amount of SUVs, and was part of a large, multi-location company.
That second dealer kept in touch and had a couple of allocations over the last few weeks, but couldn't get every feature I wanted due to constraints on certain options, so I let them go each time since I definitely want everything on it that I ordered. Out of nowhere, I got an email this week about my order being accepted, but this was from a dealer that I didn't even intend on ordering through (Dealer 3)--they simply made an error and submitted the order that I had them build and price for me in October before talking to either of the others, after some miscommunication after a staffing change.
So, given that without an allocation, the order could go several months without being accepted, you might have to order from multiple dealerships as I did--although I did not actually intend on having three different Yukons on order, that is what it seemed to take.