You should be able to manage domains in your own account in GoDaddy Pro.

I'm most likely not alone in having domains for clients in my personal account.

I should be able to manage these domains and products in the same way as 'authenticated' managed clients.

You are able to create 'clients', for clients who don't have their own account; but you can't add products on your own account to those clients.

It seems very shortsighted to think that you can't have clients who don't have their own accounts.

It seems backward to not be able to manage the domains in your OWN account in GoDaddy Pro.