I recently expanded my project/series to fit a 30 year timeline (that used to be 25). I now have 4 eras (instead of 3). Each era has multiple books. If I can get an agent and get my first book sold, I will do one book at a time till it is time to no longer make them (lack of interest, I finish the series, whatever). My first book takes place in era II. I will finish era II then go back to era I. I will finish eras three and four in succession, after era I. Reason, era I is critical to explain some big picture items introduced in era II and to make sense of the last eras.

I note my project, because I think scope is key. If you are doing a trilogy, it may disinterest people to read a "prequel" book before the closure. In my eras, each has every element of the Gustav Freytag dramnatic structure. Maybe, for large projects, it is okay to do a prequel (like the Star Wars prequel trilogy, eps 1 through 3). For 3 books, it may be a detriment to do this. I have no idea, but I wanted to share scope and how scope might be an issue if someone discusses any of this with you.