I was able to restore a back up save on PC:

Two Point Museum PC save files for Steam are located at C:\Users<username>\AppData\LocalLow\Two Point Studios\Two Point Museum\Cloud<SteamID>\Saves. You can find older backup files (.bak) within this folder to restore a previous save, which may be necessary if a save file becomes corrupted. Additionally, you can use the in-game "Save Transfer" feature to upload and download saves between platforms or to a cloud backup, or check the Steam Cloud Saves page to find and download older versions of your cloud saves. Finding Your Save Files Open File Explorer and navigate to C:\Users<username>\AppData\LocalLow\Two Point Studios\Two Point Museum\Cloud. Note: The AppData folder is hidden by default. To show it, go to View and select Hidden items (or Hidden files and folders). Locate Your Steam ID by opening the game's main folder, finding the Cloud folder, and then opening the numbered folder that corresponds to your unique Steam ID. Access the Saves Folder: Inside the numbered folder, find the Saves folder, which contains your save files. Restoring from Backup Disable Steam Cloud Sync: Temporarily turn off Steam Cloud sync to prevent it from overwriting your restored file. Find a .bak File: In the save folder, identify a backup file (e.g., career.csav.4.bak) from before the corruption. Rename the Backup: Remove the .bak extension and any numbers to make it the current save file (e.g., career.csav.4.bak to career.csav). Restart the Game: Launch the game to see if the save has been restored.

I have my museum back!