Is there a way to set only the "Full HTML" input format for the administrator role?
User #1 is a special user that gets all permissions, always. Its access privileges are not controlled by the administrator rule. You can read more about it here: Securing user #1

I haven't tried this, but the following should work: you could create a new admin user with the administrator role, and give it access to Full HTML only. If you want you can then disable user #1 completely. Before you disable user 1 make sure that the administrator role really does have all permissions. I don't know about D8, but in D7  the administrator does not automatically get all permissions when contrib modules are installed.