I may need to visit the United States briefly, therefore I want to offline backup my text messages and signal messages from my GrapheneOS phone. Once I return home I want to restore my data from the offline backup. Does anybody have experience and recommendations for this?
You are right, basically.
However, the backup solution of Signal will basically export the “content & metadata” of the messages into a re-importable format and then bundle everything into an encrypted container. You will get a separate password for that container file that you need to re-import your messages.
As far as I understand it, Signal will then re-encrypt your messages after the import with the key of your new Signal installation.