Hi Andrew! I'm not an expert on this but I asked around, and here's what I learned:
When Tableau Server is backed up, the http_requests table gets purged of entries older than 7 days. So, if your Tableau Server is being backed up regularly, even though you can set the viz filter on the admin view is to include data for a wider range, there IS no data older than that to display.
(There is an idea to make that purge on backup optional - if you're interested, please vote for https://community.tableau.com/ideas/5579)
For next steps, I'd suggest looking at the content in the Custom Admin Views group in the Server Admin community. Other Tableau server admins have made contributions there and you may find a good solution or inspiration. If not, posting in that group will definitely get your question in front of the people who know this topic best!