Yes, you should be able to sync saves from the old Mac to the new one (and back) by enabling cloud support on each. Copying the save folders over via the local network or a USB flash drive, etc, should also work (most usefully for a one time migration, rather than continuing to play on both systems).
If disabling shadows or lowering the graphics settings didn't help, that may not have been the issue with the saves.
Please email supportdos2@larian.com with a crash report, or reply/PM with a Dropbox or Google drive link, etc. These are 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.
Also include your latest save, and if older saves can be loaded, the most recent save that works and oldest that crashes. If you are using any non-Larian mods, include a list of those enabled and/or the modsettings.lsx file from your profile folder.
Each save is a folder in the '/Users/<UserName>/Library/Containers/com.larian.dos2/Data/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.
