How do I deal with the Prethoryn Scourge?

I'm aware this is an end-game crisis and it certainly looks as if it is going to end my game. My fleet consists of mainly Corvettes and Destroyers with Lasers (V) and Point Defense (III). The Prethoryns send massive fleets (Over 20k) once about a year and invasion forces biannually. My fleet, before the crisis was around 10k and now is hovering between 5-7k. I have the mod that allows me to cleanse planets of the scourge, but whenever I enter a system to do so, the fortresses/defenses they have built up just stall my troops and never die. Am I fucked or is there something I can do to actually fight back. The nearest queen is about 7 jumps away and there is over 50K worth of Prethoryns within those 7 jumps.