Uniqueness can derive from planet's position vs its star, if it's tidally locked, contents of its atmosphere in the color it has, etc. Of course you'll get repetition after a while, but if you have rule set rich enough, it won't matter that much.
Is not an either 6 rich systems vs billions of simple ones. You can have your 6, properly populated and then have the rest as an extra for exploration. With smartly made "lego pieces", you can do wonders.
Moreover, SC has advanced physics, flight model, nice combat, potential smart AIs (hopefully), so the world has the potential to be enjoyable.
Then why do this?
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Is like saying you've explored the world why visiting 6 of the neighboring towns