Well, to some extent I'd also like this to be broken -- well, super difficult -- past a couple of homeworlds. It's sort of like an uber-difficult setting, then. You can always turn on handicaps to 300% if you want to give it a go... but to your point, I just think it's not very straightforward to balance it for 8 starting home planets. One way or another it's going to be fundamentally a different game from the base game or even multiplayer, and right now it's way easier, but I think it should be the opposite given the nature of the game (and given the exploity way to get achievements).
One thing that could be done, though, would be to make it so that all the home command stations don't explode at once if you lose one of them. That could at least bring it in the direction of what you're thinking, maybe.