Same situation here. Online tutor in Physics and Maths, making half of all I earn this way.
Students' questions typically available in PDFs (or they show me the problems on their screens but prefer not to export them), frequently including drawings/graphs, needing annotation to explain sth to them. Students even send me photos of their handwritten/hand-drawn work on paper and I point out details in it.
Without being able to paste a screenshot, MS Whiteboard becomes inconvenient. Students pay for my time, so I can't play with their patience.
Undoing is also very important! Even after 3.5 years, my hand is not as stable drawing on a digital board than on paper.
I understand that I pay nothing for Whiteboard, so not much basis for complaining, but can you fellow tutors recommend drawing software for Windows with similar functionality? (I bought the cheap full version of Scrble some months ago and tried it, but couldn't draw smooth lines if they were curved or even just at an angle.)