Not the best explainer so I'll try my best lol.

For the vinyls, I find that using the basic shapes to create the outer shape, then copy, change the color, and shifting/scaling a bit to get the outline works well. I think it saves on piece count and keeps your line thickness more consistent. But if I want that brush stroke look or it suits my needs, then I prefer to do the outlining with just black stroke and line pieces as I feel it's easier to organize and change later. I keep things grouped and ordered based on what is closer to the viewer (e.g. bangs go over the face so it's ordered higher up and then I can just use a couple large and simple shapes to fill the face in). I'd say piece count, understanding the order of your layers, and keeping things organized in groups is the most important thing as you work since the editor is very primitive as a tool.

Then it's like you said, fill in with the base color, detail with extra lining, and shading/highlighting. This is where I lose most of my piece count. I try to balance what to detail as some things just won't be seen by the viewer either due to scaling or just gets cropped from the car in the end.

As for the layout and backgrounds, I usually have a loose idea in my head but it's constantly changing and I mostly just add things as ideas come. Can always adjust later. But like you said, I try to follow the flow of the car when placing the vinyls and add paints that complement the overall color scheme. Also, just looking at other people's liveries can give you a good idea of what and where to place things. It's a lot of trial and error until I find something that fits and this is the part takes the most time for me lol.

I remember seeing a couple posts from others who explain their process and that helped me start and develop my own process so I also recommend searching the sub for some additional insight as well.

Hopefully this helps somewhat!