I have thought a lot about the invader implementation. I like the rifts and the nature of the invaders as a extragalactic force.
I like, that as the rift grows, the invaders become stronger and stronger, making the fight against them harder, as time progresses.
I feel like there should be some way of fighting the invaders without conquering a all the required ancient constructs and waiting for the rift to close. At that point the fight against the invaders is won anyway.
In unlucky circumstances, I might not be finding ancient constructs and there might be rifts blocking my way to further exploration. There should be a way to close a rift locally, even if it is very expensive and dangerous, or requires the entire military of a mid tier empire. Just the possibility is important so I have something to work towards, if I'm in one of the above situations.
For example, rifts could have a stationary stabilisation platform with a large escort located in the middle of the rift. Without the platform, the rift would close slowly. The invaders could get combat bonuses when fighting inside the rift, so that it won't be easy to take down.