The Toolbox can be hidden using one of these two plugins for Krita:
The older one:
… and I would use it in combination with the patched version of KanvasBuddy by @Halcyoen. The original, and now defunct, version of KanvasBuddy is found here, and you will find @Halcyoen’s version in that topic too.
This new one that is currently under heavy development and can do much more than only show/hide the toolbox by will. I guess at first you may think that it is overkill, but it offers new worlds for your workflow: