I think we are wanting a natural explanation for events occurring before the genesis of life, both on the player’s world as well as all other life in the galaxy. At least for the general case, perhaps it would be applicable for panspermia settings. I would think that fallen empires would only exist with certain universe generation settings at initial configuration, or would be under non-LAWK option, and thus couldn’t be relied upon for all universes generated.
Also, I thought that the ascension gate would be researched and created by the player’s society, which would give it a last milestone for your society to reach, instead of being found somehow in the world. Not that a fallen empire couldn’t have begun creation of an ascension gate, perhaps you could find clues/fragments of the ancient ascension gate that optionally help speed up development somewhat.
I would suggest a change to the end of the fallen empire being an acention gate that is completed, but was built incorrectly or otherwise fails when used (perhaps it was rushed) and thus somehow ends all life in the galaxy and resets it, thus providing an even field in the universe.