You can use prolific and fast growth to take all the space before they can.
Steps:
Build empty hull (nothing on it) Upgrade it to a colony ship.
Send ship to your target, colonize, rush 1st factory, build 2 more, build shipyard.
Build Empty hull, upgrade to colony ship. put people on it.
Return to home planet and scrap ship putting people back on the starting planet.
Build another Basic ship. upgrade it,
Send to another planet... Repeat over and over and over. A new cycle for every shipyard you have. You can spam space and take it all over before any other race has a chance.
You don't even need high fertility, You can use patriotic and prevent any opponent from attacking you. It's op.