Good afternoon rich4341,
Welcome to the Apple Support Communities!
It sounds like you are having issues with your Magic Mouse since updating your Mac.
The resources below will help you address this issue:
Update to macOS10.15.7 - How to update the software on your Mac
Unpair the Magic Mouse and restart the computer. Then pair it again.
Set up your Apple wireless mouse, keyboard, and trackpad
Reset the System Management Controller (SMC) and NVRAM?
How to reset the SMC of your Mac - Apple Support
Reset NVRAM or PRAM on your Mac - Apple Support
Try disabling your Wi-Fi just long enough to see if that helps. If it does, check to see if your current Wi-Fi network is running on a 2.4Ghz or 5Ghz band. Bluetooth runs on a 2.4Ghz band, and it’s possible your Wi-Fi network is causing interference.
Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
If the issue persists, please reply to this thread and we'll do our best to help you!
Take care.