There’s a really handy test strip in this post that has helped me dial in my settings: Has anyone been cutting 0.37-0.40 material?
For better cut-through without increasing burn, I use 2 or even 3 passes.
The laser beam is shaped like an hourglass. The goal of focusing it is to put the narrowest part of the hourglass at the surface of the material, to give you the finest cuts. Changing the thickness of the material just puts a wider part of the beam at the surface and gives you a wider cut, and since it’s defocused it probably won’t go as deep. There are things defocusing is good for (e.g. smoothing out lines in engraves), but it’s not useful when you’re cutting.