I've had a couple of games in the 6-9 trillion range, but all of them became increasingly unstable around then as well, so at this point I don't really milk games anymore that way.
Here's the basic method I was using: stop your tech tree short of the 2 +10 PQ techs, culturally flip all but a corner of the galaxy, then do The Terraformer, then the 2 +10 PQ techs, put your sliders to 100% social and build up all the relevant infrastructure, then let 'er ride for awhile.
I did do the 'go back and colonize all level 10 or higher worlds' once, back when I had +15 PQ on a game that I was going to break the bank on population with (and yeah, it was tedious to be sure), but that game was totally hosed by corrupt data, never could get it submitted to the metaverse. Score wasn't that high anyway, to be honest.
Nowadays I usually flip everyone except a few and then tidy things up and end it, sometimes I don't even flip I just tech end it. I'd rather start a new game with all its unknowns then stare at a screen of a game that is already won.