I like the tax system of the game, but the population growth is quite a ridiculous phenomenon. During the time frame of the game, which is say 10 years, the population of humans rises from 10bn to 500bn. This is a factor of 50! This translates to the yearly population growth of 50^(1/10) = 1.5. In other words, every couple, no matter how old or young, has got to have at least one child every year. So during the course of 10 years every couple would have 10 kids, even those who were 5 years old at the beginning. This is quite strange, to say the least. Also, imagine starships with 2bn troopers. This would require one to have an entire floating country! If you have a 1000 people/km^2 on a ship like that you would need 2 000 000 km^2 of space on a ship like this. For comparison, this is almost one quarter of the area of the entire United States (which is about 9 million square kilometres).
This makes the entire economic base of the game seem a bit outrageous. So for the future games, I would suggest to focus the colonisation more on resources and building up the homeworld. The point of the colonisation is to obtain resources of other planets and take them for yourself. This would require corporations to take over planets and ship the resources back to the homeworld. The military part should focus more on orbital bombardment rather than actual invasions. Invasion forces should be much smaller and occupation should transfer resources to the occupying empire. The corporations should be the main part of the game and should provide the bulk of one's economy. Resources could have prices and the prices of civilian and military construction projects as well as research should all depend on their abundance. This will make players engage in a resource grab not unlike the one happening on Earth at this time.