You have made a very important point! The big picture, the end goal, whatever one calls it, must be where we begin our teaching. This is true for all disciplines. If we start with what we want students to come awat with from any lesson, unit, etc. then the route taken to get to the goal becomes much less important. Having basic familiarity with the major strands across the k-5 continuum is also essential in building a learning environment in which it is assumed that all students will learn and grow to their greatest potential.
For what it's worth, I expect that this year will be even more comfortable and productive. I'm looking forward to my 6th year using Investigations. Not only have my students come to me more prepared to succeed, but I have been able to step back and look at the "big picture" over several years, and have begun to see trends and find my own personal way to adapt activities. I have also had the benefit of teaching this program in grades 3-5, both as single grades as well as multigrade. From what you wrote, I'd say that you were further alongthe learning curve than I was after only 2 years.