Hey there, specialgardensetc.
Welcome to the Apple Support Communities! Thanks for the post about your keyboard not working as expected on your iMac. We'd be glad to see what we can do to help you out.
You mentioned that you've tried several keyboards and are still seeing what appear to be some sort of stuck keys. Do these happen in all users on your Mac, or do they only happen in a specific user account? If you haven't checked this, do so now using the steps in How to test an issue in another user account on your Mac. If the issue is specific to your main user account, there could be some settings causing this to occur. If the issue is happening across all users, you may want to check the Bluetooth connectivity. That and a few other helpful steps can be found in If your Apple keyboard doesn't work.
Let us know what you find out.
Take care