See if any of the following can help:
Delete Bluetooth preference files – Go to `~/Library/Preferences/` and move these files to the desktop (don’t delete yet):
- com.apple.Bluetooth.plist
- com.apple.driver.AppleBluetoothMultitouch.mouse.plist`
Then restart your Mac and re-pair the Magic Mouse.
Reset the Bluetooth module – On Catalina, hold Shift + Option, click the Bluetooth icon in the menu bar, and choose Debug > Reset the Bluetooth Module. Restart after that. (Sorry, I no longer have any of my Macs running macOS Catalina, but this is from my notes.)
Open System Preferences > Mouse. If it’s blank or not showing options, the driver isn’t loading. Removing the plist files usually fixes this.
Since it’s an older 2012 MacBook Pro, try resetting both the NVRAM & SMC.
If none of that helps, reinstalling the Magic Mouse drivers via the macOS combo update for Catalina 10.15.7 is the next move—it refreshes system components without wiping your data.