I happily stumbled across an update to modern document libraries I hadn’t noticed before. The modern document library “new item” menu now includes an option to “Edit New Menu” which pulls up this pane in-context:
And also includes the ability to upload a new template directly from the menu, rather than through content type settings.
Any new templates added via this method will use the default content type for that library, but provides a way to have multiple templates for a single content type.
This is great! I just did a test of this and it seems to be shared for all document libraries on one site but not to document libraries on other sites. If they could enable templates across everything, that would be really useful, but even as is it is definitely a step in the right direction.