Yes, we strongly recommend against changing Discourse’s source as it makes upgrades much more difficult and prone to breaking. Themes, theme components, and plugins should be utilized instead.
There are some existing #theme-component that may work, or can at least serve as examples if you want to develop your own.
Versatile Banner might work for you as-is or with some minor adjustment.
Showcased Categories Theme Component is a good general example of how to show custom content on the homepage.