It was indeed the texture tiling that was causing the seams.
For those that are interested, I got some good results with a 32k earth
I solved the problem using another approach:
In Blender:
-Create a UV Sphere (64x32)
-Divided it in tiles (8 vertical cuts, 4 horizontal cuts → 32 tiles in total)
-Divided all the 32K Earth textures in 32 parts (diffuse, bump, specular, …)
-Created a material for each tile with its own textures
-UV Mapped them
-Joined all the tiles back to one single Mesh
-Subdivided and smoothed the mesh
-Exported as FBX
Unreal:
-Import FBX
-Result (WIP - normals are inverted) :
