There is another way to play on the largest galaxy size and still have a large empire. You need either:

1. invest in max system level modernization -> Autonomous Adminstration building (system no longer counts towards overcolonization)
2. use Benthic Gems in system modernization to wipe away overcolonization penalty

If you want to make use of these in a timely way, you have to make do with whatever luxury you can get the most of and spam upgrades fast (even if it's trash like glitter or super-potatoes). Plus get those resource extraction building techs.  Also a reason I always set resource scarcity to Abundant.

Still a pain in the ass if playing as the Cravers though, as you'll probably conquer much faster than the other races. The days of the early-access Craver-Jesus setup are long since over I'm afraid (iirc laws didn't cost constant upkeep then).