If you found an online course titled "Well-Rounded Software Engineering," what would you expect it to cover? (I'm starting to build a course with that as its working title.)
This article actually covers a lot of what I was going to suggest. Note how it's almost completely language-agnostic, and that's a good thing. "Well Rounded" means you're looking for a very general outlook, a jack of all trades's view, of SWE. So you can't lock your subject matter into any one language, paradigm, platform, library, etc. That leaves you with the soft/technical skills of engineering as a whole, not just SWE.
So the more technical skills would be Analysis, Critical Thinking, Problem Solving and Troubleshooting, Communication and Presentation, the scientific method, etc.
The softer skills are more social or environmental: Working in a team, Giving and handling criticism, etc.