How Can I Archive a Single Text Message Thread From My iPhone or Mac?

I would like to archive/export/save one very long text message thread for safekeeping. It can be from my iphone or my mac – both have access to the same thread. Currently all of my messages are saved into iCloud but I do not trust that methodology long term.
I don’t mind purchasing a 3rd party app if that’s the best way.
My wife has a very long thread on her iphone that is of very sentimental value to her. I would like to be sure it will not be lost.
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Rob Mayer