Booking hostels/travel in advance

Hi, I (28M) am leaving my job in a few weeks and will hopefully be solo travelling for a few months, most likely in Asia.

I land in Tokyo on April 10th and then have seven nights booked at a hostel, before three nights booked at Kawaguchiko, and then four nights booked in Kyoto.

My provisional plan is then to head to Osaka, Hiroshima, and Fukuoka before heading to Busan, and I was planning to sort bookings for these hostels (and all transport) once I’m out there to give me a little more flexibility on dates/locations. However, I’m conscious that this is peak time and also Golden Week starts at the end of April. I’m just wondering how far in advance I should look to book hostels/transport during this period? Any help would be greatly appreciated.

TL;DR - how far in advance should I look to book hostels and transport in April/early May in Japan?