I think the occupation victory was a good idea in Sins 1 but also that it could be more balanced if it wasn't just one victory planet but a handful, scattered around the centre of the map. The blue print I have in mind here is Age of Wonders III's seals victory which had a few elements to make it exciting:
- As mentioned, it was typically a few map markers of interest (seals) that would generate victory points rather than just one. This would give all/most players a stake rather than exclude those who just happen to be on the other edge of the map and practically unable to interfere otherwise.
- Those were all towards the centre of the map and pre-determined. So it wasn't a turtle victory where the player could build some victory structure and fortify around it. You had to go out and get there. In some instances, your realm could get big enought to encompass one but rarely enough to secure victory in itself.
- The seals were not only heavily defended, they kept spawning new defenders while being occupied. So, it wasn't enough to just conquer them. You had to keep an army there and thus tie up military resources that could instead be useful for general empire defense or offensive campaigns.
One of the consequences of this design was that it mitigated end-game drag. A strong player could push for control of enough seals and force weaker players to go all-in to contest those seals. It could still take a few turns to finish but tended to be more tense than in the typical run-up to a 4X end-game victory.