Reservation needed for train to Kushiro?

I will arrive at Narita on 8 November, and fly to New Chitose on the 9th, then take the train to Kushiro.

I am thinking of buying a JR pass, but this will mean I cannot buy a ticket until the day before I need to travel.

Will that be fine (very unlikely to be a crowded train) or is it better to not use a JR pass until I leave Kushiro - having a tivated it at Narita).

Many thanks for your advice!

Yes, starting this spring, the trains to Kushiro require seat reservations.

I am thinking of buying a JR pass, but this will mean I cannot buy a ticket until the day before I need to travel.

What JR pass? A Japan Rail Pass? A Hokkaido Rail Pass?
I believe it is possible to make online reservations for trains in Hokkaido on the JR East reservation website for both the Japan Rail Pass and the Hokkaido Rail Pass before you arrive in Japan (but be careful about where you can pick up the tickets):
https://www.eki-net.com/en/jreast-train-reservation/reserve/wb/RouteSe...

The same website can also be used to buy tickets for the train to Kushiro without a rail pass.

Note that the train to Kushiro departs from Minami-Chitose Station (not the airport).

Also, if you plan on using a Japan Rail Pass, and you purchase it through the official website, you can make online reservations before you arrive in Japan:
https://japanrailpass.net/en/

Will that be fine (very unlikely to be a crowded train) or is it better to not use a JR pass until I leave Kushiro - having a tivated it at Narita).

Regardless of whether you use a pass or not, you will have to make seat reservations. From that point of view, it does not make much of a difference.

If you are not worried about the train being crowded, then the easiest thing to do would be making the seat reservations at New Chitose Airport station.

Make sure that whatever train pass you buy it actually pays off. You’ll need to do quite some travel in a few days to make them pay off. Otherwise a single ticket is cheaper.