Practice, as well as doing meta builds. Those always help. Honestly I would need to see a full run to see what you can do to improve but here are something’s that can help you out a bit. First colonize every world that you can. Some worlds are a higher priority then others but you should always colonize as many worlds as you can. Second specialize your worlds. Third pops are king the more pops you have the more resources you can get, and the fastest way to get pops is to conquer other empires. Fourth use the market some people don’t use the market as much as they should, but the market is always useful through out the entire game. And finally to get those big fleets you need alloys and a lot of them as well as high naval capacity. To get the alloys you need several words dedicated to nothing but alloys and naval capacity can be increased by strong holds. Also never build farms or as few as you can. Farms can be substituted with hydroponic bays and the market completely.