An earlier point about the mid-game: Stellaris is at its best at both the early game and the late game, when a galactic crisis starts. In most other 4X games, the end-game is just mop-up time, where the match is already decided, but the crisis can alter the whole landscape. Currently, the earliest it can happen is 200 years in, which in my opinion is a bit late though. (Then it's rolled every five years, if it doesn't happen immediately.) The coming update will introduce a slider for this too, though.
Also, a fun fact that I recently learned about Awakened Empires (having been vassalized by Jingoistic Reclaimers): some twenty years after they are no longer being challenged (by either a crisis, or them not being the strongest empire yet), they'll start growing decadent. They'll start losing their bonuses, and given enough time, will start accumulating penalties to the point of not being much better than the Fallen Empires that they used to be. So if you can survive and build up until that time, you should be able to beat them and become independent.
Also, a fun fact that I recently learned about Awakened Empires (having been vassalized by Jingoistic Reclaimers): some twenty years after they are no longer being challenged (by either a crisis, or them not being the strongest empire yet), they'll start growing decadent. They'll start losing their bonuses, and given enough time, will start accumulating penalties to the point of not being much better than the Fallen Empires that they used to be. So if you can survive and build up until that time, you should be able to beat them and become independent.
