Hello soupspoon,
Thank you for using Apple Support Communities and for sharing these details with us. Sounds like you're experiencing syncing issues when trying to use OneDrive with your iMac. It also sounds like you've done a great job in troubleshooting this issue. Uninstalling and reinstalling the app was the best approach to take. If that did't help, we can recommend a couple of more steps you can try to help further isolate this issue you're experiencing. Follow the steps below:
1.) Test your iMac using a different Wi-Fi network. This can isolate possible network connectivity issues.
2.) Install any pending updates that might be available for your iMac: Update macOS on Mac - Apple Support
3.) If you're already fully updated, try booting up in safe mode. This step can help isolate possible software related issues: How to use safe mode on your Mac - Apple Support
Intel processor
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
Depending on your results, follow the steps under “If the issue continues in safe mode” or “If the issue doesn't continue in safe mode”.
If you continue to experience syncing issues with OneDrive, consider reaching back out to Microsoft. They would be the best suited to assist you with their application: OneDrive help & learning
Thank you for using Apple Support Communities. Take care.