Custom race: Get trait Colonizer. And get fast, along with more military production.
Settle planet, and get Pragmatic Level 1 3 free constructors. Convert constructors to colony ships.
Put production into military 100%. No research or wealth.
Use the colony ship you built and the 3 constructor/new colony ships to colonize planets. 6 planets on turn 9. And thanks to Colonizer (free first building) you will have a shipyard on each one.
Easy Peesy.
Set all of these colonies into building colony ships and by turn 20 you can have like 15 colonies or more depending on star system distance and number of planets on each one. All of them will have shipyards thanks to "colonizer," meaning they can produce even more colony ships. You'll be growing like an exponentially increasing bacteria.
Thalan Tech Tree is best for this because Hives is awesome. It's cheaper than a factory (important for rush buying) and provides an automatic 4 boost to production regardless of the planets' population (which with this strategy is going to be very low). Later on the Gaia Vortex and the Hyperion Matrix gives big boosts to ALL of your planets regardless of its population.