Thank you for your feedback!

Unfortunately, I can't choose the dates for my holiday (in any case, I think Spring is the best time to visit Paris)

SJUAMMF. I visited Himeji, Okayama and Kurashiki on my first trip (I used Hiroshima as a base)

I have checked out timetables on Hyperdia. So, as a first draft:

Day 1. day of arrival (early morning), trains Narita-Tokyo-Sendai, walk around city center.
2. boat tour to Matsushima Bay / visit to Zuigan-ji
3. Hiraizuimi. Sleep in Tazawako or Kaukunodate
4. Kakunodate
5. Nyuto onsen area
6. Trains to Kanazawa (via Tokyo? 7.30 hours -flight from Sendai? 6 h)
7. Kanazawa
8. Trains to Fukuoka (flight?)/maybe same day ferry to Busan?
9-10-11. South South Korea (Busan area). Ferry back on day 11. Train Fukuoka-Kumamoto
12. Kumamoto
13. Mt Aso (possible in 1 day from Kumamoto?)
14. To Tokyo (flight: 4 h/ trains: 8 h)
15. Hakone (self-guided)
16. Leave Japan

Seeing possible time spent on trains on days 6-7-8, maybe it's better to skip Kanazawa and try to fly from Akita or Sendai to Fukuoka.
Or stay in Tokyo 2 nights and then visit Hakone. (6. train to Tokyo / 7. Hakone / 8. train or flight to Fukuoka+ferry).
Or perhaps change Kyushu for more time in the north coast...
Ufff, I'm confused.

My plan is to take a JR Pass for the full stay and, if needed (I did it last time), an ANA airpass with 3 legs.

Apart from anything else, maybe it's a short time in Korea, but I prefer spending most of the time in Japan, a country that is fascinating me. I just want to have a little taste of Korea, and maybe plan a wider trip to Korea sometime in the future.

Thanks!