If you select Story in the main menu, and then Load Campaign, is the save list empty in the load window?
Check the save folder to see if the saves are there, but not showing up in-game for some reason. If you switched OSX user accounts, that could account for the saves not being listed, but in that case Galaxy shouldn't have just synced the profile and not the saves.
Each save is a folder in the '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/PlayerProfiles/<ProfileName>/Savegames/Story' folder.
You can try searching your hard drive for *.lsv files, in case the saves got moved somehow.
You can disable cloud saves for the game in Galaxy. A failed backup shouldn't remove the local saves, though, or syncing if there are no online saves, I would think.
I don't suppose you have any backups of your Documents folder after the play session but before restarting the game? That would require a conveniently timed scheduled backup, or a backup or sync program monitoring the Documents folder.
You can try recovering the (presumably) deleted saves, but the more you use a system after files are deleted, the less likely they can be fully recovered.
https://www.ibeesoft.com/recover-data/recover-deleted-files-on-mac.html
