This empire is built around the new mercenary enclave mechanics which was introduced in the Overlord dlc. This build is optimal for playing a very tall militaristic playstyle, which uses fleet power instead of pops and systems to acquire power and wealth.
This playstyle is usually done through creating tributaries. See my Grand Fleet and Void Dragons builds for other tall military empire builds,. The merx is able to go even taller through also being a megacorp that can install branch offices on client empires and earn dividends from maxed out mercenary enclaves.
The key elements of this build are the Naval Contractors and Franchising civics of a megacorp and the Clone Army origin.
Naval Contractors increases the cap on mercenary enclaves. It also raises naval capacity by 15%. Franchising removes the loyalty penalty for having multiple vassals. It also gives a bonus to loyalty for vassals and reduces the empire size due to branch offices.
The clone army origin allows us to have fantastic admirals, by taking the clone ascendant option.
The huge resources we can acquire through multiple subsidiary vassals and mercenary enclaves powerfully mitigates the clone army’s hard cap on population growth for their primary species.
All the bulk of this empire’s pops need do is man fortresses for naval capacity.
There we go, the tallest of tall military builds, the Merx Condottieri. Without googling, do you know why I named them that?