iMac cannot find my wireless mouse. I've tried all suggestions from Apple and on Google. What else can I do?

You can use diagnostics to test your hardware: Use Apple Diagnostics to test your Mac - Apple Support

That may or may not reveal a problem. When this happens to me, and it does every now and then, I use my wired USB mouse and to open BT and forget and reconnect a device. That's always worked for me and we have five Macs (two MacBook Pros) we use the Magic Mouse on.

Other than that, once I reach my max frustration point I just reinstall the OS and move on. Takes about 40 minutes and has never failed in eight years on various Macs. Some see that as an extreme remedy; a restart, about six clicks and wait 40 minutes is just puppy time for me. Of course that's just me ... you have to do what's right for you.

Magic mouse won't connect on iMac - Apple Community

Fix: On ... System Preferences / Network - removed the blue tooth drivers. And then added them back into the Network panel. Applied the changes. Magic mouse was then visible and connected immediately upon clicking consent dialogue.

The following will require any USB keyboard and mouse. If you don’t have them, borrow or buy them, they are inexpensive!

1.     Connect the USB keyboard and mouse to the computer.

2.     Open System Preferences – Bluetooth

3.     Un-pair the wireless keyboard and mouse.

4.     Turn off Bluetooth

5.     Restart the computer in Safe Mode per the directions in Use safe mode on your Mac - Apple Support

6.     Restart the computer normally.

7.     Open System Preferences – Bluetooth

8.     Turn on Bluetooth

9.     Re-Pair the wireless keyboard and mouse.

10.  Unplug the USB keyboard and mouse and begin testing the wireless devices again. 

same thing happened with me. First the new mouse has worked, after got find the own mouse, and it was working too. But later -about one hour later - either the new or the old mouse not found anymore.