Need to make reservations in advance?

Good morning! Wishing you a lovely week. My friend and I arrive in Japan on 1/11 and will be traveling from Tokyo to Shin Aomori on 1/12 and then onward from Shin Aomori to Hirafu, Hokkaido on 1/16 via train then onward to New Chitose to fly out on 1/19. If I buy a Japan Rail Pass, I can't make reservations until I arrive in Japan and activate my pass. Do trains tend to fill up? How likely is it that we will be able to get seats on the 1/12 train to Aomori and the 1/16 train to Hokkaido if I can't make a reservation until 1/11? For the security of dates/travel times, would you advise that I just buy tickets individually online now so we can be sure to have reservations?

Thanks so much for your advice!
Be Well,
Rachel