We used https://docs.unity3d.com/ScriptReference/Texture2D.SetPixels.html
to draw brushes on Texture2D. Generally, I had a quite few problems, and one big is that it's not really performant but that was the best solution I could think of.
Drawing texture should be moved to GPU(but I had no enough knowledge and time to do so)
Actually, because of painting we had to somehow force and warn people to not resize window. It causes that painting doesn't work properly :P