The scope of your project seems to be way above what the traditional tools of Unreal Engine can offer. I suggest you find or create your own tools that doesn’t use nav meshes.
million kilometers on different islands.
The further you stray from the origin of your level, the more inaccurate calculations will become.
Here is some further clarification regarding the terrain sizes: UE5 - World Partition - Massive Map Sizes? - #6 by noone
The basic rule of what you will require for a computer for main memory and video memory for importing and loading all cells for various world sizes is:
8km x 8km = 32GB memory and RTX-3050 8GB
16km x 16km = 64GB memory and RTX-3080 10GB
32km x 32km = 128GB memory and RTX-3090 24GB
64km x 64km = 512GB memory and RTX-8000 48GB
According to the calculation, assuming everything scales linearly, you would need a PC with 16 Petabytes of RAM for about 1 million KM in size. This also assumes you will end up using the partition system.
Could you clarify why you would want to do what you are trying to do?