Johan, that's a lot of ships. Wow!
Thank you! My ships are small though (Escorts-Frigates-Destroyers) are only ~400 in size. Capital Ships and Carriers are maxed out, though.
After doing the math, I notice you have a space port at 100% of your colonies. I only have 2 spaceports in this game. One at my homeworld, and one at the captured enemy homeworld (Varunat, in my screenshots). Could I be gimping my spaceport income by doing this?
I highly doubt it. I only do that because it feels like it's the "right" thing to do (role-playing wise). Spaceports are a big drain on resources and maintenance, with questionable strategic importance in far-flung colonies. I use them to control pirates more than anything else. They can, however, add to your colony's happiness, which allows you to tax them more (even though corruption will negate most of it.)
The way I look at it, a spaceport costs me about 6K to maintain. If the colony's tax income is not that much, then it's not worth building. However, I try to put at least one spaceport in each system. After all, why settle a system if you don't plan to build infrastucture there? In that game, I have plenty of income to handle the spaceports though, so I build them everywhere.
I believe the private sector is somewhat tied to spaceports as well. It will try to maintain X number of freighters per size of spaceport, and all those suffling of resources may generate extra cash for the economy.
Anyway, it sounds like you're well on your way of solving the problem. Maybe now you can try a few different things and let us know how that work out. For example, it would be interesting to copy your current game save, and continue in two different paths (one with just the 2 spaceports, one with as many as you can afford) and see how the economy compares after a few years.