Hi Paul,
I could fully understand your feeling.
I have tested "Edit in Excel" in Safari on a Mac, I couldn't reproduce the issue. To further troubleshoot the issue, could you try the following information?
1. Did you use the latest version of Safari? Can you try using the Private Browsing mode to see if it helps?
2. Could you use another browser, such as Chrome or IE browser?
3. You mentioned it stuck after the "Were opening workbook in Microsoft Excel" pop up, could you try selecting the Excel file and clicking the Edit button like below? Because this step will open the worksheet in Excel client.
4. I'd like to confirm if downloading the Excel file to your desktop works well. Please check it by going to the library in SharePoint Online ->select the Excel file ->click Files on the top section ->Download a Copy.
5. Does the issue occur in another application when you edit the file in SharePoint Online, such as Word or PowerPoint? Does the "Edit in Word" work well?
Feel free to let us know any testing results. We'll provide further assistance based on your updates.
Thanks,
Alice Wang