@DELTA_12
Sorry to wake up this thread but I’m confronted to the exact same issue you have for our own project which takes place on a space station in low orbit around the earth, which means at an altitude of ~400km above the surface of the planet.
On my side and from my experience in the movie industry, you can’t skip the texture resolution factor and detail maps won’t help. I’ve made a little Houdini asset that simulates the situation and shows the actual portion of the planet seen from that altitude. Its very tiny and on a 2K game screen and from my calculations, you must use, for the entire planet, a map of 64kx32k (32k16k would almost do but on our side we also anticipate 4K displays too). Anything lower and you will see pixels or blurred pixels, nothing sharp. This, of course, if you want a dynamic planet which actually rotate and change light condition (the station is orbiting the planet so the lighting varies quite quickly). If not, most games then use skyboxes with static matte painting of just the portion seen.
At first I’ve experimenting by simulating UDIM and splitting the planet in several UV areas but this led to a very heavy Material yet with seems artefacts when using shared texture samplers, artefacts I don’t have when using map texture samplers but then I needed more than 16, which is the limit.
I finally find the solution, sorry if that sounds like an advertisement but this is really the only solution we found for that kind of situation: we ended up using the Granite plugin from Graphine which is able to stream huge textures little tiles by little tiles (mipmaps included), and by using a set of 64kx32k textures (created from NASA/BlueMarble).
The only other solution I can think of is to split your planet piece by piece and create different materials for each piece but… this will be very heavy and I’m not even sure about not having texture seems in this case.
Did you actually solved this issue on your side ?