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It would be easier if you use a “Search Posts” module to extract a list of all your Wordpress Posts, their IDs, and Created Date into a Google Sheet. Add a “Last Shared” column.
Then, use another scenario to Search Rows by “Last Shared” empty or ascending to get the next oldest post that is unused/unshared.
At the end of the scenario, update that item’s “Last Shared” with the current date.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!