Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.
You can back up just the save folders, the profile or the entire D:OS 2 DE folder. You may also wish to backup your entire My Documents folder (at least) and restore it after reinstalling.
