Additional meeting and call-related features and issues within Microsoft Teams for business
Yes, it is not a final solution.
But I'd rather try this way when I give lectures.
About desktop. You can enlarge your Keynote window to the whole screen before sharing. This will not let others see your things on your desktop nut almost only Keynote.
I am not quite sure but even when you share only Keynote in a slide show mode, does it also take all your screen?
Of course, if you keep on doing multitask while having a meeting, this is not a good method.
At least under some conditions for some people, better than nothing.