My request is essentially the same as listed here: Toggle between WYSIWIG/Markdown editor mode with keyboard shortcut but that feature is closed, and I don't see any open features that match it.
I prefer viewing and editing primarily in wysiwyg - it's basically the upsides of the viewer combined with the ability to edit quickly. However, often I need to drop back into Markdown quickly to muck with something, and there's a handy button for doing that in the upper right, but no keyboard shortcut for it unfortunately.
Right now I'm just toggling between the markdown and viewer, but it's a lot more swapping than I'd like.
If this is accomplishable through a plugin, I can try and write one to do just that, but I'm new to Joplin as of today.