I'm on good terms with most of my neighbors and control key choke points into my empire with citadel star bases, so nobody is going to touch me. My main fleet is about 20k in size (off of one starbase!) while my opponents average about 5k fleet sizes. Even faster if I have research agreements. My research speed increases my research rates to about 2.5k research points a month, and because I have such a small empire, I can research most techs in an average of about 8 months. I have about 50 branch offices and make about +1k research of each type. When I setup branch offices, the very first building I make is a research lab which gives (+6,+6,+6) for all three research types. Likewise with just about any other resource in the game, but with different ratios. I can trade energy for minerals at about a 1:1.1 ratio, so if I need 10k minerals, I can pretty much convert my energy credits. I also managed to get the galactic market on my homeworld, so the trade fee's are reduced by 10%.

Every two years, I am maxed on energy credits. I'm now at the point where I get about +1000 credits per month and my resource cap is at about 42,000. A lot of the time, the planets have a high population already, so I can queue up like 3 different branch office buildings. This gets the system out of my admin pool and 'expands' my commercial empire. The moment I gift a populated planet, I also setup a branch office on it. After I win the war and take their systems, I gift every system to my neighbor with a commercial pact (via the trade/diplomacy screen). They *must* have adject hyperspace lanes, just a shared border isn't enough. To 'conquer' an enemy, I go to war against them and make sure that they're neighboring an empire I have friendly relations with and also a commercial pact. I could have had a leaner empire, but I really just wanted to snag a lot of the nearby choke points and rare resources. I have about 5 planets and have a system level of about 200, and an admin level of 120 (with tech upgrades). Over the course of time, that planet will grow and become very valuable and the energy cost to setup a branch office will pay for itself, many times over. Get a branch office going as early as possible, even if it yeilds +0.1 energy. The faster you can do it, the less likely other corps can get them from you. Be the first to establish branch offices on your neighbors.

Stellaris megacorp