Start with building Neo Habitats and Power Stations on every planet preferably in lines (the same types next to each other), you will need these resources to be able to build other types of buildings. At early stage i recommend building 1 Drone factory for faster bulding and 1-2 Defense Batteries for automatic defense.

If you have that start to build as many Science Labs as possible. These will make the Research part of the game a lot faster, which is required to unlock new buildings, levels and an overall stronger ship. When you researched Steel Citadel start to build it instead of Science Labs as this gives you twice the amount of research points with the same area coverage.
When you researched everything destroy the research buildings(leave 1Science Lab and 1Steel Citadel so you can still access any levels) and build another 2 Defense Batteries and a lot of Trade Hubs for passive income. You can also build Harvesters to areas where nothing else can be built.

There are limitations, you can only build 2buildings at the same time on a planet and only 29buildings are allowed(but you can make it 30 if you start to build two buildings the same time when you have 28, this will also enable you to slightly go above 100% Population Employment and Power Load). To get more building you can destroy Power Stations and build Tesla Reactors instead. When you finished the main game and installed all Updates you want, you can also destroy the Drone Factories for more space.

With all 9planets fully covered like this you should get 3000Credits/5seconds. But since aliens will often attack you and that attack on any planet will stop the passive income you will get around 1million credits/hour. You can spend this money on Items, like 10-10 Screen Bombs and Quick repairs for easier gameplay or buying Core Mods to level up faster.
Feel free to leave any suggestions in the comments.