I think the current state of multiplanet spawn lead to too many unfair battles. Don't you think everything could be more balanced if this choice was made from the lobby and "public" (or at least raw numbers on how many on which) ? It would prevent a guy from spawning all alone and getting a significant eco advantage on others.
I think the bigger problem is that the small moons that "loners" spawn on can have a huge amount of metal for their size. The metal density slider will allow people to make systems where even if someone is alone, they will have a headstart, but not a huge eco advantage. Plus part of the fun in multi-planet games is finding out where everybody is.
I made a thread that dealt with a way to fix this issue a while back. https://forums.uberent.com/threads/suggestion-limit-number-of-players-on-planets.58091/