We have already solved this problem/feature in several projects with the following module:
https://github.com/mediarox/module-checkout-placeholder
It uses a so called "LayoutProcessor" to change the address fields on PHP side.
The principle can be seen in the documentation in the chapter "Add custom fields that influence other Checkout fields":
https://devdocs.magento.com/guides/v2.4/howdoi/checkout/checkout_custom_checkbox.html
The "LayoutProcessor" copies the label string into the placeholder variable ("placeholder" property of the field).
