~agroschim wrote an amazing post about generating fantasy maps: https://grenzland.club/~agroschim/blog/2026-04-19_Fractal_Planets.html
And here's a treasure trove of alternate earths mapping material: https://zenodo.org/records/10659104 check out 3c. Global Elevation Source Maps.zip in particular!
edit: more maps: https://zenodo.org/records/10659112