I think VS Code is one of the best editor/IDE for Inform 6 right now. (Disclaimer: I actually don’t code in I6 that much, and I’m the maintainer of the VS Code Inform 6 extension.)
Regarding VS Code, there’s the inf6langtools extension that adds an object/class tree to navigate the code. In my very limited experience of it, it might mark some valid code as errors, but I suppose it depends on the project.
In case you haven’t seen it, there’s also the IF Player extension that allows to open and play Z-machine and Glulx files directly in VS Code.