You need to have Visual Studio + Unity Tools for Visual Studio. Just saying that explicitly in case you only did 1 of those.
After that, adding it as the external editor, as you said you did, is the only other thing that is required, as far as I know.
My experience with troubleshooting Visual Studio and Unity is pretty much non-existent. I’ve simply never had it bug out with the way I install them. I start the installer for Visual Studio Community, select the entry for game development with Unity, deselect the Unity editor from the installer, and let it install. After that I simply install the release of Unity I want.
I know this isn’t very helpful but it’s literally worked for me every single time following these steps on multiple systems.