Disable Steam Cloud and make sure to back up the data folder:

On the startup screen there is a white button labeled "View log file." When clicked, it opens the file explorer in the Valheim data folder. Go up to the parent folder, "Irongate" and back up the "Valheim" folder.

It's in the data path that any backup program should be backing up, so if you make daily backups, there is a good chance you can recover your character.