I don't think there any videos out there for creating starmaps with Cosmographer.

There's not any tool that will populate the map randomly with systems, you'll have to do that manually.

My most important tip for new users is to make sure to understand the program properly before jumping into a large-ish custom project. As you said, it can be a bit overwhelming at first, so if you haven't done so already, I highly recommend working through the basic tutorials in the user manual, even if they aren't about space and sci-fi.

That list shows the fill available in the current map. If you want fills from another style you need to import them into the map first, then they will be available to pick from that dialog.

None of the tools do that automatically as far as I know. You'll need to manually place each node. Turn on Snap and it should be real easy to place the nodes precisely.

When CC3+ renders a map to an image, it does so in sections. It does this because there usually isn't enough memory to calculate the entire image in one go. This value controls how many pixels each such pass should contain. The default value is on the lowish side when it comes to modern computers, so increasing it allows us to export the drawings in fewer passes, and the issue you were having usually occurs on the edge of each pass.

Once you've entered this command, it is the new default until you set a new value, it is not a per-map setting. And it is a good setting for everyone to use unless you have an old computer with very little memory (below 8 gigs)

I don't see any reason why they have to be 10' wide, unless they at some time had to be widened to that point to bring large objects through. In the real world, quite a lot of underground passages are narrow things, not even 5'.

No, trace won't work on entities on frozen layers

The PPI setting is a meaningless value outside of print calculation. Just completely ignore it and always just concentrate on the width/height in pixels.

The important thing to check is that both height and width is set correctly, since CC3+ will crop the value if necessary to retain the ratio. (Well, technically, all you have to do is to get one correct, and just set the other one to a really high value, since it will be capped at the max allowed by the aspect as long as the rectangular section is specified correctly.)

When doing the rectangular export, for perfect precision, I recommend turning on cursor snap so you don't have to align them perfectly manually.

To make sure the export is done correctly from CC3+, you should be able to check that the pixel dimensions are correct from the file properties in Windows (or by opening it in an image editor)

To hide things from your players, I would consider putting things on different layers, then you can reveal one layer at a time as they explore the area. You'll have to predetermine which chunks to reveal of course, but this is a good idea revealing things piece by piece.

For elevation, as JimP suggested, contours are the best. Since you are using Mike Schely Overland, there is a contour tool called Height Contour in the :CC2MI: catalog. It looks white int he preview, but it is transparent, and you can lay multiple on top of each other to lighten it bit by bit.

I would consider one of these approaches here:

1. Join paths to polys and create a multipoly of your floor before putting it into perspective. The disadvantage with the multipoly approach is that it is difficult to edit after the fact.
2. Split up the original trace into separate polygons for the rooms and corridors. This gives you a lot more surfaces, but it is easier to handle and edit. This approach will require a bit more processing for your trace before turning it into perspectives though.