You should listen to the "Kingcast" podcast where he goes into detail about the pilot and long term plan He spent 3 years on mapping everything out and he did an excellent job laying out his vision and the decisions he made. Including exactly why he chose to start at that point. Really worth a listen: https://podcasts.apple.com/us/podcast/the-dark-tower-with-glen-mazzara/id1512844649?i=1000475938916

The sad and ironic thing about your reply is that the actual Dark Tower pilot, produced for Amazon, shot and apparently well received did NOT get picked up for series by Amazon. You know why? Focus on LOTR and budget concerns. This is a great read up, it'll make you sad https://ew.com/tv/glen-mazzara-canceled-dark-tower-series-amazon/