Hi Uroboric. Welcome to the forums
There's no precise answer to these questions. There's always more to know, but you don't need to know everything—nor could you, indeed.
If you don't feel your designs look good enough, it's worth having a graphic designer to turn to. Likewise, it's good to have one or more people to turn to in any area you don't feel you are really expert in. It makes for nice collaborations, and takes some of the pressure off you, too.
You seem to have the front end pretty much covered if you know HTML, CSS and JS. That's pretty powerful in itself. Having a backend language is even more powerful, of course. I don't know how useful Python is for web design—but perhaps having that under your belt would be the edge that you need to stand out. (Most people would start with PHP, or perhaps Ruby.). Having an 'edge' or special skill is a good way to stand out, so if there's something you are particularly good at or that you really enjoy, it's worth getting known for that.
Good luck, anyhow.