Seat reservations during high season

Hi I am considering buying the the JR pass during our trip to Japan in 2027. We intend to arrive around 1st April and stay for 17 days. I can see you can book seats in the train stations, but during the high season will booking a seat a day or 2 in advance be too late as trains are likely to be fully booked ?

Can you prebook seats in Tokyo for all my train journeys throughout my stay or can seat only be booked from departure train station?

Thanks