That's true about the delivery time - I just bought an X5 (same factory, same line): ordered Sept, delivered in Nov, BUT there are a few gotchas to the process (which I'm intimately familiar with

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After your order, BMW could change options - while this appears to be over now, back in Q3/4 2021 many vehicles were delivered without touchscreens, trailer hitch option, rear seat pulls, no wireless charging, no wifi, no soft-close doors, no heated/cooled cup holders and a whole raft of other potential - but small? - misses.
So, while people did get their vehicles relatively fast, there is the potential it'll be missing something you wanted (though, again, post-order changes seem to be over for now). My vehicle is missing the wireless charging & the rear seat pulls in the cargo area - neither was a huge deal for me & I knew it going in, so I went ahead with delivery.
In case you weren't aware, I would say the competitor to the Navigator is more the Alpina XB7 if one is comparing top-of-the-range like to like; but it's priced significantly higher.
With that, the reason I have a Navigator L on order is because, like you said, the X7/XB7 is really a slightly larger X5 versus a body-on-frame full size SUV that the Navigator (and escalade and grand wagoneer) is. Size-wise, the X7 seems to more compare to the new Sequoia and some of the other newer large mid-size SUVs coming out.
If you do want an X7 the key is to find a dealer that has an allocation available - ideally one going into production in the next few weeks. The way it works is, dealers are given allocations monthly and if they don't have a customer for them they config them however they want and place the order. If a customer shows up before the vehicle goes into production ("status 150") then the dealer can re-spec the order to your config.
So, a given dealer may not have any allocations for you this month, but wants to lock you in for next month's allocations ... versus another dealer who either has an allocation waiting to be spec'd or one going into production next week they can update. Thus
delivery timing is 95% dependent on the dealer you pick and the allocations they have (and where those allocations stand in the production queue).
The vehicle takes about a day to produce, but from status 150 to completion it's about 2 weeks (though, as I said 21Q3/4 there were some glitches to this). Then, depending on where you live, delivery can take anywhere from a few days to 3 weeks for, say, West Coast deliveries. BMW has a train depot right in the factory so once the vehicle is done it either is put onto a train or truck for near-state deliveries.
If you happen to live within 500 miles of the factory and you find a dealer with an allocation about to go into production you could get your vehicle in 3 weeks which isn't uncommon. If, on the other hand, your dealer doesn't tell you the whole truth about their allocation statuses (some like to lead on customers, placing higher margin customers in the queue before others) it could take 6 months or more.
In my case,
I validated my dealer's allocation and production timing (this is easy to do with BMW as you can request a production number & then validate the production week yourself!), but I also live on the west coast, so it took 10 weeks.
If a dealer won't give you a production number there is no allocation! Unfortunately, some dealers take advantage of people who don't know BMW's process; once you have that production number you can log into BMW's website (mygarage.bmwusa.com), assign it to you, and check the config. Via the 1800 you can call BMWUSA and independently confirm the production date.
Once you know how, finding an allocation with an acceptable production week and tracking the status is fairly easy: there's a 1800 number to call & a web site that tracks each step.