Mission accomplished.

So to recap, I had forgotten I had initially installed a template that included a lot of mods I wasn't aware of. When I uninstalled the template these mods were retained and cause the error I was experiencing.

to remedy this as per @barco57's guidance, I exported manufacturer, manufacturer_info, tax_class and tax_rates. As a precaution, I also created a full export of my db as well as my site files. I then deleted all the files and emptied the db and loaded everything from stock and imported the db tables I had previously exported.

Now when I click to add a category I get the new category form as expected.

I am so done with non-stock zc stuff it's not even funny anymore...