Should we buy JR Pass ??

Hi! My husband and I will be in japan in a month.

We are planning to stay in japan for 3 weeks (very excited! )
Osaka - Kobe (round trip)
Osaka - Kyoto
Kyoto - Nagoya
Nagoya - Tokyo

These are our itinerary and not sure if this is worth buying JR pass.
We want to travel by bullet train between cities. Do you guys think we should buy 21 days pass?

also if i decide not to buy jr pass- where can I reserve bullet train??

please help meee :(
thank you so much

Kind regards
jane