I've just finished Burnt Offerings, and my players are all really interested in renovating and claiming Thistletop as their stronghold. In fact, in regards to the question of why they would want to do that, they all stared at me and asked why they wouldn't want to. Which is a compelling point. They're also doing some interesting work with NPCs that is resulting in them developing into interesting potential cohorts.
However, it seems like the timeline of the campaign as written is very fast--the Swallowtail Festival is the 22nd of Rova (September), and then I'm supposed to be starting Hook Mountain Massacre by early Kuthona (i.e. December): The chapter introduction starts with "A few months have passed since the events of the fateful Swallowtail Festival." So this doesn't give a lot of game-world-time for construction, or for development of NPCs. I'm thinking of spacing it out a bit--maybe doing the Feast of Ravenmoor module in Gozran (tax season, naturally), then having the Skinsaw Murders occur in the heat of the summer to really play up the corpse stink. Then I could move on to Hook Mountain for the next winter, or even the winter after, depending on how far into the construction/downtime rules they want to get.
Has anyone done this? Does anyone have any thoughts on it? It seems like the forces at play here have been scheming for years--millennia, in some cases--so it doesn't seem like a stretch that they'd be playing a slow game.
The timeline doesn't matter all that much to tell the truth. As you said, the involved parties have been planning a long time. A few more weeks or months won't matter.