What are the essential factories to have?

Hi, I'm fairly new to the game, about to finish phase 3, and I noticed that I haven't built many permanent factories.

So far, most factories were focused on producing one specific item (like space lift parts or motors or something), but always due to to a momentary need.

I realize I haven't setup any proper factories (besides my coal and fuel power plants) to mass produce basic items, and find myself needing to build something new everytime.

Since I've started setting up a train network around my world, it seemed like the perfect time to build all the essential factories I'll need (or have been needing) before starting phase 4.

However, I'm wondering what items to prioritize. Should I focus on more basic resources like ingots or plates, or more refined items like frames? Should I deal with the overflow at each step of the process or just at the end?

I know a lot of it it purely personal, but I'd like to hear what you think about this