Could the transparency issue be avoided by using premultiplied alpha?
To use premultiplied alpha in Godot, you can create a CanvasItemMaterial and set the blend mode to PMAlpha.
Not bad for a basic whiteboard, but doesn't seem to have undo functionality?
I tested about a dozen free whiteboards before, and found only https://limnu.com/ had a good solution for function and ease of use. Have used it for tutoring before.
Edit: Looks like it may not be so free anymore, in which case, thanks OP! Other alternatives I also found before: https://www.groupboard.com/products/ and http://stoodle.ck12.org/
I began my training regimen and only spent a few days in it...
I am ready and willing to continue learning this language, even more so to converse with my bretheren!
OMG SO MUCH YES! https://limnu.com/slackwhiteboard/