As for access, you’ll need to decide what takes less space. Managing who can access each project or drawing with project or drawing meta data, or managing what drawings or projects a user can access with user meta. You may be able to leverage roles and capabilities to minimize the data stored. It depends on how fine a control you need for the various elements.

I was considering a custom post type for the Project with an array of drawingfile names in an associated post_meta array, but the number of drawing files could go up to thousands.

I’m still planning on using a post_meta array for team members with varying read/write access, etc, but that should be a smaller list.