Figured it out. Appears to be a bug with PKCanvas view. The issue is not present if you test on a device, in release mode, with the app running standalone (not connected to XCode). Took me a while to figure this out but I have been testing it for a week with small strokes, large, multiple strokes added quickly and still no crash. Apple, please fix this? annoying.

Hey, I found a solution but it's not ideal, but enough for me. In Mission Control, uncheck "Displays have separate spaces". This seems to work fine. Then to disable the fact that all other screens go black when pressing the green fullscreen button, use this app called "RightZoom" to reroute that button to be used just for the old-style stretching window to fit the screen. Haven't experienced any issues with Xcode since then. Otherwise, with "Displays have separate spaces" enabled, XCode crashes for me every time i open preferences, and also at random points it freezes every 10 mins or so. Works with latest XCode version as of 26/03/2021. edit: using MBP 13 from 2018, no touchbar