Earning income in this game is a headache in and of itself. Beyond the things mentioned above relating to war weariness seriously fucking with your ability to collect taxes, in turn forcing you to possibly capitulate and pay up in wars you could actually otherwise win, just because you cant win it immediately. Growth is painfully slow for planets if you tax them, so the quickest way to earn more taxes, is to let your homeworld give you income while you create a shit ton of untaxed new colonies, while researching techs that help them grow and funding space stations in orbit of them to further that along. Your homeworld can provide enough income on it's own for a while and since growth is exponential based on pop size, it can cap out it's pop while partially taxed which you need it to do sooner rather than later. The only worthwhile structures you can build on a planet's surface to make it any way better, are limited to 7 or so wonder buildings that can only be built once by only one empire in the game. Besides that, the only buildings for planets are things that allow recruitment of certain troop types or provide a slight defense bonus. Nothing like farms to grow crops or entertainment centers to make them happy, etc. That stuff either comes from the space station in orbit, or the one shot wonders (if you manage to get any first), or not at all.

On top of that, once you do finally grow out your other planets you get to learn the hard way about this shit mechanic known as corruption. Corruption is supposed to increase the further away from your capitol planet, so that distant colonies have their potential income penalized and can't be as powerful of an income generator as your home planet. This mechanic is way too high though and planets right next door to your home system can have their income capped by as much as 40%. Further away and it can be 80%+. You unlock 2-3 extra regional capitols you can build once that drops the corruption on the planet theyre built on to 0%, and supposedly reducing it in the area, but again, if it's not on the planet itself, it goes high very quickly once the planet is at max pop. This is with the corruption modifier slider set at the lowest setting at the start of a new game, so expect default values much higher.

This translates to you the player only having 3-4 planets earning over 75% of your income from taxes on a map with 1400 stars and an empire of 50+ well developed planets. This is one of those things that doesn't seem to hinder the enemy AI much, though they are terrible about growing their planets to begin with and tend to tax their new colonies to the point where they grow abysmally slow throughout the game. Somehow they still make shit tons of money. As a player, non maxed out planets might earn you 5-10k in taxes if they have a few billion pop, below that, they earn you practically nothing. A maxed out pop planet of decent size and quality might earn you 15-45k in taxes, a third to half of which can immediately be eaten up by the cost of the spaceport's maintenance in orbit, which is necessary.

So all in all, the entire game economy revolves around you closely guarding these select few planets and if you lose one, your empire will fall apart. Sure there is income from trading and such, which can be quite high, but it is irregular, unreliable and easily interrupted by wars, trade sanctions, pirates or juts random events happening in the background you will never be privy to.

Also you cant  move or rebuild planetary capitals/regional government centers.