At least with my setup, drawing with a pressure-sensitive stylus in Mypaint does not work while screen-sharing in a Zoom call. It is so sluggish (in the sense of recording a new line position only every 50 pixels or so) that it is unusable. The problem may or may not happen with other screen sharing software as well. So I'm happy both for a solution for Zoom calls, and for any solution using an alternative tool.
The issue does not happen while screensharing is paused in Zoom, so it is not connected to the X server compositing done by Zoom on top of the Mypaint window (that is, the colored border around the screen and a small call window without decorations). Rather, it seems connected the CPU and / or GPU load of capturing and / or encoding the video stream.
Details about my setup: ThinkPad X201 Tablet, using the Wacom tablet integrated into its screen for pressure-sensitive drawing. Ubuntu 20.04, or rather Lubuntu 20.04 but it should not matter for the purpose of this question. Zoom for Linux in the version current as of 2021-06. Mypaint in the current version.