I don't know why I never thought of this before!
There I was stuck in one corner in the largest galaxy map (scattered, tight clusters). At first, it wasn't too bad, as I had no competitors for the few planets there with me. After that, though, there was a wide swath of empty space and I knew the colony rush was going on w/o me in the rest of the galaxy and I was basically screwed.
Well, I had previously spotted two wormholes right close to the homeworld and had guided the flagship around them so as not to "lose" it for a long time.
So, I build colony ships with one engine and everything else life support except that on each ship I added a survey module.
The survey module let them through the wormhole! Presto, I was in two distant locations in time to grab planets!
A little later, after the colony rush had ended (I had habitable planets on uncommon), another wormhole opened near a planet and I repeated the exercise but this time with a constructor.