When you reinstalled the game, did you make sure the install folder was empty/deleted before reinstalling, or install to a different location? You would most likely have wanted to move or otherwise back up the 'Projects', 'Public' and 'Mods' folders in the '..\SteamApps\common\Divinity Original Sin 2\DefEd\Data' folder; there is a 'Mods' folder (at least) in the '..\Data\Editor' folder, as well (probably safest to backup everything left after uninstalling).
If applicable, disable Steam cloud support either globally (in the client click on the Steam menu and select Settings, and then Cloud) or just for this game (in the library right click Divinity: Original Sin 2 and select Properties, then switch to the Updates tab and check the Steam Cloud section).
Alternately, exit out of the Steam client and just start the game directly from the executable when required.
Next, try browsing to the '..\Documents\Larian Studios' folder and rename the 'Divinity Original Sin 2 Definitive Edition' subfolder.
This folder contains the saved games, configuration files and a level cache folder. Deleting or renaming it will get the game to recreate it on startup; playing the game from a different Windows user account would effectively do the same thing. With Steam running and cloud support enabled, the client would just download the cloud copy of your existing profile.
After that, see if you can start D:OS 2 DE. If so, create a new profile and maybe check that you can start a new game and save/load. If that works, exit and copy a couple saves from the renamed folder into the newly created profile's ..\Savegames\Story folder. If that continues to let you start the game and load the saves, move the rest of the saves over.
If the game still crashes, delete the new My Documents D:OS 2 DE folder and rename the original back again.
If that gets the game working, try preparing one of the default GM campaigns to see if that works. If so, try starting the editor, and see if you can create a test project and see if you have any problems.
If the default campaigns are ok, try copying your own campaign from the renamed folder into the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\GMCampaigns' folder, etc.
If necessary, there is a way to effectively reinstall the game without having to redownload it.
Right click on the game in the library and select 'Backup Game Files...'. When that is done, right click again and 'Delete Local Content...', then in Windows Explorer, check the install folder and if it isn't deleted/empty, rename it (before deleting, right click, select Properties, switch to the Local Files tab and select 'Browse Local Files...'), then restore the game backup.
If that gets the game working, check the editor, and if that is fine you can start sorting out your projects from the renamed folders and moving them back. Hopefully that will get them working, as well, but if not you may need to do more troubleshooting to determine what exactly is causing the problem.
