Stage 5 can give you a huge boost in writing complex queries!
You say that like it's a good thing. I like my queries simple.
Also - the stuff you have under "stage 6" should all be learned before "stage 2" in my opinion. Knowing how to write efficient queries is far more important than group by / join / etc.
I imagine it depends on use case. Anything more advanced that subqueries/CTEs is largely the domain of ETL developers (not exclusively, obviously) but most others using SQL don't need anything that advanced.