Buying JR pass vs separate tickets?

Hi!

Just wanted to run my itinerary by an expert to see what the best option is :)

Here it goes:

Day 1-5: Narita Airport to Tokyo, all around on Yamanote Sen, Yokohama, back to Tokyo
Day 6-8: Tokyo to Nagano, Nagano to Narita Airport

I'm debating on buying one of the many options of JR passes or buying a separate one way ticket from Narita Airport to Tokyo on the first day, 2 one way shinkansen tickets (Tokyo - Nagano, Nagano - Narita AIrport), and perhaps all day tickets for each day sightseeing in Tokyo.

Also, if I bought the JR pass, would it cover all of my travels?

Any advice is greatly appreciated! Thanks in advance!