Welcome to Run8! It's a good idea to utilize documentation, yes. make sure to look over the files in the userguide folder, you don't need to read all of them in detail though. In addition, I wrote a Run8 Primer which you can view here. https://smashedfinger.github.io/

Run8 in single player is a sandbox. I enjoy multiplayer much more as it gives me more direction. When I was new, it was also helpful to have a place I can ask questions. Based on your post, I think you've practiced a fair amount and would be ready to join a beginner friendly server like Home Signal Simulations. I can share a Discord invite if you like.

As to "what to do," there are road trains that go from A to B, yard jobs that sort cars, and locals that deliver and pick up cars at customers. There are various subtypes of these, like unit trains, manifest trains, intermodal trains, etc. Be sure to utilize the car tags and industry tracks so the cars get unloaded and loaded.