Is there a way to batch-edit metadata in the Mac OS Photos app?

To reframe you question, you need to understand what Photos really is. It is essentially 3 things :

• previews you can look at on the screen
• your originals sorted somewhere (including EXIF information, related to the shooting, and possibleIPTC information, vulgarly referred to as "metadata" )
• a database to handle the previews

The database does not handle other metadata than the one you mentioned (and favorites). Unlike what Aperture used to do. So no, there are not built-in tools to do that.

You could come up with third-party tools that could modify your originals, but you would not be able to see it through Photos interface.

Although I do not have any personal experience with the program, MetaImage seems to be what you are looking for. From their website:

Metalmage for impeccable metadata

MetaImage is the only macOS tool that allows to edit, read, and write metadata while working with all types of tag formats. Navigating through a sleek interface, you can change everything about your image metadata — and apply the same edits to hundreds of similar photos.

Works alongside your Photos app

If you own a Mac, you own Photos. This uncomplicated photo manager comes with lots of improvements on macOS Catalina — like organizing images by collections. So yeah, you’re gonna need it. With MetaImage, you don’t have to do any syncing to change metadata for images from the Photos app. There’s an extension for that.

Just to be clear, I have no association with the publisher of this program and have not used it personally.