Hello, I usually have a general plan that is define a few months before leaving. My first destination is usually where I can fly cheaper to from Europe to either a new country or one I like as a hub base for travel like KL or Singapore. From there I can fly to anywhere in Asia for cheaper and only a few hours of travel.
During planning first I try to select country I want to go to again and new countries I want to see and start doing a general routing, I might by the first few flight tickets in between countries to facilitate visa/visa exempt process and leave the trip inside the country blanck so I can adjust, just doing my entry town and probable exit down, I might follow a direction south to north or west to east or something just make my way slowly around in order to make it to my flight out of the country.

Nice I love Japan, haven't been there since 2019 but probably will come back at some point, enjoy your trip, my new one is coming to an end as I plan on flying home before Xmas. Will do more posts about the trip, I have a lot of private backlog journal that need to be posted.