Over building is not a good idea as you discovered with the space ports. If you run on harder settings you could not afford them anyways, so maybe that makes the game easier . Until I get filthy rich, I will protect new space ports with some small fleet. Once I am filthy rich I don't care anymore (and they are usually built faster in a mature, rich, empire).
It is not much need to mess with mining station designs. If you itch to do something 3 extractors can be useful.
Blockades are stupid in the game. The Atuuk have a certain low relation to your empire, which makes them want to blockade your port with the rather broken mechanism.
Trade sanctions is something else the AI wants on empires below a certain "like" level. Sanctions will annoy the other empire somewhat, and block trading. Blocking trading is good if you want to mess up an empire with shortages of something you could have traded them, and bad if you need to get something from them.
If you got 15k carbon fiber with your flag on your space ports, you don't have a shortage, but you could have shortages in certain spots. If nothing are build, it is probably something else missing. Building something will make the station reserve the needed resources. If stored amount is lower than reserved amount, you got a shortage on that station, which will block construction of some components. This is perfectly normal trying to build a space port on a new colony, you have to wait for freighters to move in resources.
The resources with foreign flags are resources booked by other empires. A freighter should be on its way to pick them up. (should does not always imply will).
Gizureans are a fast breeding insect race. People migrate, and you will see aliens migrating to your colonies unless you set restrictive population policies. The only time this matters is if you are at war with the empire of the same race as your populations. This will make the people more or less angry depending on each race. Annoyance reduces happiness, as do high taxes. Very unhappy populations start rebellions. As or the gizzies, since they breed fast, they are more prone to arriving as immigrants.