There is only one factory, and it also makes the Highlander, Grand Highlander, and Lexus TX. The Sienna would need to become a lot more profitable for Toyota to reduce production of the other 3 to make room for more Siennas.
They could raise the price if they wanted it to be more profitable! Dealers already exploit the long wait list to do so in a way that doesn’t pass through to Toyota.
I’m in a large metro area; when I go to the Toyota website and look at inventory within 20 miles, I see 1 Sienna that is not in transit and not sale pending. That’s across 9 dealerships! By contrast, for the new Land Cruiser I am seeing 31 vehicles that are not in transit and not sale pending. On dealer lots. Most of them have $3k+ discounts listed! 12 Corolla hybrids; 24 Camrys. 87 Tacomas!!!
I have no idea what Toyota’s strategy with the Sienna is.