Hi Qualitymix, welcome to the Gibbon community! 
The initial templates included in the Reports module are fairly simple and currently don’t include displaying multiple sets of grades. What we’ve discovered is that there’s rarely a typical report card, almost every school has their own needs and requirements. For this reason, the templating system was designed to be as flexible as possible, so that people can edit the templates to suit their schools needs. It does require some HTML and Twig templating syntax, but it’s still much easier than editing PHP code.
The best way to get started editing templates is to go into the Manage Assets section of the Template Builder, then duplicate an existing template and edit it. Be sure to visit the Twig docs for more info on the template syntax. There is a preview button when editing the template to see what it looks like. It’s best to have your system set to Development mode when working on templates, as this will disable the caching system. Otherwise, you may need to clear the report template cache in System Admin > Cache Manager (v21) for your changes to be visible in any generated PDFs.
We do eventually hope to have more example templates and more documentation about this process. Our capacity is limited and currently focused on refactoring some areas of the codebase, but it’s something we’re certainly aware of and working towards.
Here’s a link to a previous post about adding markbook grades to the report card: Mark book to Report Cards. It would be possible to code a system like you’ve suggested, but its currently beyond our capacity. If it’s something you’re interested in developing, or finding developers to create it, we’re happy to offer pointers to help ensure it’s a change that would be mergeable into the core.