How to restore purchase information into a de-googled device?

This is covered in the FAQ, this is not possible.

License purchased on Google Play are managed by Google and require Google Play. You can usually side load Google Play on that device.

There is a solution for degoogled device How can I pay for symfonium without google play? - #2 by Tolriq but no migration as it’s very manual and actually cost me money in the long run to offer this.