how do I save a lengthy text message string

To save an entire message string (aka conversation) you need to use an app like iMazing or TouchCopy. They can save, export & print entire text message conversations including photos and other attachments.

To save ALL without doing it one conversation at a time you will have to use to third-party software such as iPhone Music, Messages & File Transfer for Mac & PC · iExplorer (macroplant.com)