Normally, I would strongly recommend reading AARs to see how people do these things. Unfortunately, at the moment that’s somewhat infeasible due to the demise of the old forums where most of these were found. You can access some content through Wayback Machine but unfortunately not all of it.
A recent AAR which is still accessible is Blue Emu’s Sirius Business. It’s less of an AAR and more of a playthrough with stream-of-consciousness write-ups, and I will immediately caveat this recommendation by saying that the author often makes statements based on inaccurate or outdated information, but it is a good read to see some examples of how to deal with a variety of situations (notably Raiders).
I wouldn’t bother colonizing unless you see a good reason to. Eventually it makes sense to start moving stuff to low-cost bodies that are easy to grow for purely logistical reasons (e.g., moving factories to be closer to the main sources of minerals), and there’s benefits to having specialized colonies for financial centers, research, etc., but you don’t have to colonize every sub-2.0 body in sight just because it’s there.
PDCs are no longer in the game. You can build armed space stations (which are just ships without engines) or surface-to-orbit weapons instead.
One useful resource may be the Aurora 4x Documentation Archive put together by another user here. This seems to contain most of the changes in the C# version, which is a useful basis to understand most of the game mechanics (e.g., diplomacy is in there somewhere).