Is it possible to pay to have an Odoo employee execute the migration script on the on premise server? I remember I heard about that but I don't know if it's still an option.
These approaches should be completely flipped, the default should be your upgrade engineers get remote access to the customer's database (vpn, etc) and run the standardized and ad-hoc upgrade scripts on their system. Shipping the customer's data back and forth for every upgrade is absurd and a privacy nightmare.