...Once the Festrons appear on any of your planets, the game is over for your race.
No, and not even for that planet. Since only Core world populations grow now, all you need to do is rush build a colony or constructor ship to deport the murderous Festron to some forgettable class 2 hellhole Colony world or even an orbital economic starbase.
If you catch the infestation right away, you can prevent what happened to you.
I have been playing as Krynn; they start with a random mix of any species in the game. I have restarted a few times due to bugs and such, and I've dealt with a Festron in my starting pop mix more than once now by building a colony ship and sending them off. (One time I even dealt with two this way, and had plenty of time to do it! They didn't murder a single being.) Once they're off your Core world you have nothing to worry about.