Thanks for your posting! The built-in report templates are very flexible and you can implement a lot things with the standard reports and some things might not be obvious to find. I'm happy to suggest possible implementations if you could share a few more details on what you are trying to accomplish.
We usually don't recommend developing custom reports and using the built-in templates instead if possible, or use the XML exports or API to generate custom reports outside of TestRail. Custom reports on the Reports tab would only be supported on TestRail Server but the built-in reports already support a variety of different use cases and implementations (possibly in combination with the exports).