Hi G Lambert
I tried multiple times to bring a correct answer here, but Cohort Analysis is unfortunately not in my skills.
I'm raising this thread hoping our community can help you
Lénaïc RIÉDINGER, Global Community Engineer Tableau
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One quick thought is that these numbers and data look fairly manageable in size so you could export a csv report from GA into a CSV file and then with that data source and your excel data source, create a join.
It would probably be quicker and easier to manage. I utilize a a custom report out of GA each month that I bring into a directory and use for a wildcard union join for reporting and it's pretty easy (and quicker) than attaching to GA each time.
I hope that helps.