You can download previous versions of the whiteboard app https://www.filehorse.com/download-microsoft-whiteboard/. There are instructions for how to install a previous version earlier in this long thread, From back around November 2021.
I agree that the new version of the whiteboard app absolutely sucks for use as a whiteboard compared to the really good older version. At the very least they should have put some sort of mode choice into the app so that you could have it work either the old way or the new way.
I hope they will get enough negative feedback again that they will have to revert the app again. Meanwhile, I have disabled automatic updates in the Microsoft Store so that they won't slip this really bad new version of the whiteboard app into my machine behind my back.