With a bit of playing around, I think I have the best setup.
Main species
Prepatent subspecies
.
Civics Fan Purifier/Mining Guilds Ethics Militarist/Fan Xenophobe.
Mining Guilds/Industrious and Ingenious combined with invading the primitives gives you an explosive start, and clearing 5 tile blockers to get 5 new pops early on also contributes a lot. The trick is to use all that unity you get to beeline not for Supremacy but for Adaptability's +1 building slot bonus. With that you suddenly have enough room on your planets for 3-5 more alloy foundaries than anyone else, and you have the minerals to fuel them. This easily doubles if not triples the navy you can field. Obviously just ignore bureacracy/research entirely until you've defeated at least 2 empires and absorbed their dudes, and make sure to disable most of their newly conquered buildings to get them working the energy and minerals you need to sustain increasingly large navies. If you let those pops promote to specialist you've basically ruined your game. You'll get tons of unity for this and be able to fill out supremacy quickly, Switch on Fortify the Border to get more starbase cap when you need it, I was pushing 160 corvettes by 2230 and beat a GA Advanced Start empire that I didn't even realize was Advanced Start until I took their homeworld and realized it had 80 pops on it.
EDIT: Be sure to check that the right pop (non-necrophage) is always the one growing. It seems to sometime pick the wrong one for the homeworld at the very start of the game and for new just-founded colonies. Just flick it to forced growth of the non-necrophage and then un-force it and it will reset to the non-necrophage and stay there.