So, after a lot of failing tests (again) including different techniques, I finally got something that I quite like.
I'm trying to go from regional map to end with the world map, in order to get quality for regional maps & to not mess with scales (not sure it will works, but well...).
I'm starting with the big island of Osininka, east of Norumbega.
Osininka.jpeg
I've just done a small part of the shaded relief. You can guess it takes some times (even more after 5 failures...).
I'm not sure it fit the scale (at this scale, 1 px is supposed to represnt 1 km).