Unless BMD has temporarily deactivated eGPU support in the early betas for v15 (or changed how the app works with GPUs in general), both the standard and Studio versions should support that eGPU setup on that specific OS. The tests I've run all indicate that the main difference is that the Studio version allows you to select more than one GPU to process the tasks in your workflow.
So the standard version shouldn't prevent you from selecting that specific GPU (RX 580) in preferences, only prevent you from selecting more than that GPU. The studio version should let you use both the internal and the external GPUs at the same time. That said I don't know if it's working in the latest beta. I haven't tested it since Beta 1 and in general mostly v14.x.