Can you create a new profile, start a new game (on story difficulty) and save/load? If that works, try skipping through the ship as much as possible to get to Fort Joy, and see if you can save and reload there. If that works, exit, restart and see if you can still reload the Fort Joy save.
Try lowering the resolution and graphics settings, and switching to Windowed display mode, to see if that will make a difference. In particular, try disabling shadows; that avoided a crash reloading Fort Joy saves reported recently on a MacBook Air.
You can try updating to the latest Catalina 10.15.5 beta version. There are some driver fixes in the latest beta which might help, and Intel is currently looking in to the crash in the drivers when reloading Fort Joy saves, mentioned above (two people have also reported a crash trying to join or create a multiplayer lobby on a MacBook Air recently, one helped by the beta, who was already playing in windowed mode, and the other who tried the beta first, but still crashed until they disabled shadows and vsync).
Apple has a signup process to be able to get
access to the betas. I'm not sure if there is an estimate for the public release.
If a new save and multiplayer is fine:
Are you shutting down all non-essential programs before starting the game, especially anti-virus and any backup programs that may be monitoring the Documents folder?
Try doing a clean boot and then test the game. Start/restart the Mac and hold the Shift key down as soon as the progress bar comes up in the startup window, releasing it after the desktop appears. Alternately, from the login window, hold the Shift key when you click the Log In button, and release it when you see the Dock.
If the beta or lowering the settings doesn't help, please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc). If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file from your profile folder.
Each save is a folder in the '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/PlayerProfiles/<ProfileName>/Savegames/Story' folder.
In Finder you can zip a file or folder by right clicking on it/them and selecting compress.
Also include a crash report (collected in the 'Console' application, under 'User Reports'). Alternately, the crash reports may be saved in the '~/Library/Logs/DiagnosticReports' or '~/Library/Logs/CrashReporter' folders.
