Travel 3rd Jan18 Hakuba-Hiroshima JR Pass

My family and I are planning to travel from Hakuba to Hiroshima on the 3rd Jan2018 on JR Railpass.
From Hyperdia we are looking at taking the following:
-14:37 departure from Hakuba on LTD EXP AZUSA 26 to Shiojiri
-then LTD EXP SHINANO 18 to Nagoya,
-Shinkansen HIKARI 481 to Shin-Kobe,
-Shinkansen SAKURA 573 to Hiroshima arriving 21:36

Also we are planning to travel Hiroshima to Osaka on 4th January departing:
-19:23 from Hiroshima on Shinkansen Sakura 566 to Shin-Osaka
-then Ltd Exp Konotori 25 arriving Osaka at 21:09

My concern is that we are travelling on two of the most busiest days in Japan. Does anyone know if we will have problems reserving seats on these days as we are not able to prebook with a JR Rail Pass.
We are arriving in Tokyo on 20th December and plan to exchange our vouchers and book our seats for the above journeys.

Appreciate any advice or suggestions as I am particularly concerned about the Hakuba to Hiroshima as this is a very long journey if we have to stand all the way.

Thank you in advance.
Susie :)

Not sure what type of answer you are hunting for. Your question seems to answer itself.

It will be busy. You will have trouble booking reserved seating. Probably not all, but most.

Best advise to to be flexible with which trains to take, and start moving earlier in the day to allow for this flexibility.