The transitions for the D-C-G progression and its various permutations i.e. G-D-C ("Knockin' On Heaven's Door") aren't hard. You just need to put in the work.

The Fundamentals course is a foundation. A pretty easy basic one (compared to the one by Christopher it superseded) to facilitate a sense of progress. If you don't learn those few conventional open chords now, you'll regret it later.

As to the question about future possibilities. Later you'll discover and use all manner of variations.