Is JR Pass effective?

Hello! I would like some help to see if it's worth to buy the JR Pass with my itinerary. I will be staying in Japan from December 01st to December 28th. I will be going to:

Dec 02 - 06: Okinawa
Dec 06-10: Osaka and Hiroshima
Dec 10-15: Kyoto and Nara
Dec 15-18: Nagano
Dec 18-21: Takayama/Shirakawa-go
Dec 21-28: Tokyo with a couple of day trips.

I wanted to get the 21 day National Pass, but I am afraid it will not be enough because of the number of days and also because it might not be cost effective.

Do you have a sugestion?

Kind regards!