I just tried out a new strategy for the third time and it works for me (small to medium maps, crippling)
The idea is inspired by something Wyndstar once wrote about giving the enemy defenses so he built weaker ships.
You give the Drengin a production/and or science tech - let them pay with whatever (you may have to make peace for a turn or two.
Then the AI starts upgrading buildings and slows shipbuilding (it takes double time now). Then you strike again and the AI will be hard pressed to build military and will not build OFMs.
I guess this does not work on really huge maps - in small maps you can wipe them out before they recover and can use the new buildings.
Positive side effect: If you have a 1/99/0 slider setting - you don't have to manually disable civilian improvements when you need ships at the front, because some have almost been built.