We have a feature called Themes that allows you to launch a new project and create a new Prismic repository that is automatically filled with your defined Custom Types, is this. what you’re looking for?
At the moment this is as you’ve seen, only possible on the Slice Level with Slice Machine projects. The only way to update your full custom type is to copy and paste the JSON like you’ve described.
I would also love to see this made available on the custom type level, so I’ll add another tick to the feature request tracker. Although for the moment this isn’t something that we are working on.
If/When we decide to implement this we’ll update you here and in the GitHub issue you referenced.