I’ve thought of this too. There is likely a CLI command that can speed this up - but not in the UI.
Update. Ah, here it is:
https://meta.discourse.org/t/creating-a-category-with-permissions-via-the-api/34870?u=tobiaseigen
A feature I was missing recently, that you’re reminding me of, is the ability to create a handful of categories that ‘inherit’ the settings and permissions of other categories. And to mass apply settings and permissions to existing categories to make sure they are all the same.