For regular applications, the easy bypass of that warning has been to right-click the app and click Open. That specific sequence (i.e. you have to do it through the contextual menu) allows display of a dialog box that has a button that I think is labeled "Open Anyway". Does that also work for .pkg/installers like this one?
You can also use the Terminal method of course, but I am not a command line type of person.
As long as you downloaded Exiftool from a trusted source, bypassing the notarization warning should be safe. ExifTool is extremely well known, credible, and widely used for years. It is even embedded in many photo apps. If it is you will see it mentioned in the app credits.
And as others have said, to do the basic job of fixing one photo's EXIF there are many many Mac apps that provide a friendly GUI front end while running Exiftool in the background or something similar. Two examples are the aforementioned xnViewMP, and A Better Finder Attributes, but there are many more. Some free some paid, most notarized.