Necrophage, Organic Processing, whatever you want for your second civic besides Devouring Swarm/Terravore; though I like Divided Attention + the Imperial Prerogative perk for Cybernetic Ascension with this build because it allows unlimited feeder colonies at no empire size penalty. Intelligent plus Natural Sociologists if going Genetic, or Natural Engineers and Unruly or Slow Breeders if going Cybernetic. The extra unused trait point will be necessary for Cybernetic and neither of those negative traits will have a significant impact in the early game or later on as a Cyborg Necrophage Hive.

Make your Prepatent Species non-Lithoid, Incubators, and Charismatic. With Charismatic Necrophytes, even without Ascetic you can provide all the Amenities you need with Necrophytes/Evaluators/Synapse Drones without any Maintenance Drones.

Completely purge your Guaranteed Habitable Worlds to remove Stellar Culture Shock. Then move some of your Charismatic Incubator Livestock there. It's important to do it in that order, and you may even want to switch to Extermination Purge for the last primitive pop.

With Organic Reprocessing and Incubator Livestock, you'll be able to rush more Alloys in the early game for more colony ships/starbases/other ships. Colonize every planet you find. Even if it's low habitability for your Livestock, you can grow more of them there and they still produce the same amount of Amenities.

Optional for Cybernetic Ascension: Take Unyielding and Synchronicity first. Rush the requirements for Cybernetic and make it your third tradition (you still have halved pop upkeep from Necrophage). Complete the special project. Modify your species to be Durable, plus whatever other Cyborg traits you want. After the first 20ish years you should have well over 100 of your Prepatent Pops if you did colonize everything. Then switch your Prepatent species from Livestock to Undesireable and Necrophage them for a tidy sum of ~10,000+ Unity (from Synchronicity). Reform your government to swap out Organic Processing and finish the rest of the Cybernetic tree with that Unity. Even without a replacement Necrophyte species, you'll still have decent pop growth and assembly, more pops/colonies/starbases than everyone else already, your pops will have barely any upkeep, and your pops will be much more efficient.