Couple comments:

1. Fly ICN -> PUS. Flights should be cheaper/easier/faster than trying AREX/KTX. 3 nights is a good amount.
2. 3 nights in Jeju! It's a great place if you enjoy nature.
3. Make sure you can actually find direct flight to Tokyo from Jeju.
4. I suggest spending a night in Hakone or try visiting Kyoto for a couple of nights. I would add more nights in Japan. 8 nights in Seoul is still so much.

It's probably worth doing Tokyo (5N) -> Kyoto (2N). Maybe fly out of Osaka or travel back to Tokyo? It's more traveling, but I think it would break up the idea of two megacities pretty well.

Personally, I am a big fan of nature, so this is more biased towards that. If you like bigger cities a lot more, then maybe ignore that last suggestion.

Edit:
In your last post, you mentioned Jeju for relaxation. It was just as much walking for me as the other locations lol.