24 day Trip to Japan to Korea – Having problems planning logistics – Need Help!

My girlfriend (23F) and I (24M) are planning a last-minute trip to Japan and Korea from Dec 18 – Jan 10. We’ve both been to Japan before (Tokyo, Osaka, Kyoto). We want to shop for clothes, look for good food and just wanted to hit up new spots that we haven't been to before. We aren't planning on bringing much so we can fill our bags with clothes. Still flexible with some flights (Korea + Hokkaido) so the itinerary is adjustable.

Our only booked hotel is Richmond Hotel Premier Asakusa International from Dec 19 – 22 but we are still deciding for everything else.

Our current rough plan is to:

Tokyo (Dec 19–22): Shinjuku, Harajuku, Shibuya, maybe DisneySea, shopping, casual exploring.

Seoul (Dec 22–28): Seongsu, Hongdae, Itaewon, Gyeongbokgung, Gwangjang Market, optional DMZ/Incheon trip. Busan if we can fit it. Would most likely be a split between Seoul/Busan.

Osaka (Dec 28 – Jan 3): Odawara, Osaka (Dotonbori, Nara, Kobe), Kyoto (Fushimi Inari, Kiyomizu-dera, Gion, Arashiyama).

Hokkaido (Jan 3–8): Sapporo, Niseko ski day trip, Otaru or Noboribetsu Onsen.

Tokyo (Jan 8–10): Yokohama Cup Noodles Museum, TeamLabs Planets(maybe), other animal cafes, kimono tea ceremony.

Here are some concerns:

• We wanted to spend Christmas in Korea and New Year’s in Japan but are worried there there will be closures, which might make it difficult to enjoy the trip?
•Is it worth skipping Hakone to get more Tokyo/Korea time?
•Any tips for last minute logistics with flights, day trips? We probably will not get a JR Pass as it might not be more affordable than individual tickets.
•Anything we’re overlooking that’s a must do in this timeframe?

Current Flight Plan:

  • Dec 18: LAX → Narita, Tokyo (Booked)
  • Dec 22: Narita -> Seoul, Korea (Not booked) (Zipair – One way?)
  • Dec 28: Seoul -> Osaka (Not booked)
  • Jan 3: Osaka -> Sapporo (Not booked)
  • Jan 8: Sapporo -> Tokyo ( Not booked)
  • Jan 10: Narita → LAX

Currently Is this the best way to plan this trip logistically? Want to be as time efficient/cost efficient if at all possible. Any advice / general tips are much appreciated!

An alternative we were thinking is if we started our trip by flying to Korea right as we land in Japan on the 19th.

by Time-Eggplant9572