You can do this with Message Filters + an add-on called Shedule Filters. That add-on name really is spelled that way (misspelled).

Normally message filters only run automatically on new messages, but it is possible to tell Thunderbird to run filters on a particular folder, at any time.

The add-on Shedule Filters allows you to schedule filters to run regularly.

The "matching" condition for the filter can be set for Age In Days is more than, and the number of days you type in.

Installing an Add-on in Thunderbird

Bruce Johnson's suggestion is probably a helpful reply for Windows users, but I have a Mac. Unfortunately, apparently "Shedule Filters" is not available on Mac.

So when you look at the page, it says it is not available for Mac? Shedule Filters

Since I use Windows, I cannot see that info.

So the process will not be automatic for you. You can still create the filters, then run them manually on the folders. You can add a "Run filters" button to the Mail Toolbar, to make the process easier.

Correct, that add-in page says "Not available for your platform."

Essentially I'm looking for something like MS Outlook's Auto-Archive feature, which is essential for the sustainability of all corporate email worldwide, AFAIK. I find it odd that I'm apparently in a tiny minority of Tbird users who need that.

I suppose this does explain why I stuck with POP for so many years beyond its lifetime of general acceptance.

Thanks, Bruce, for the Mail Toolbar button workaround suggestion.

perhaps this add-on https://addons.mozilla.org/en-US/thunderbird/addon/autoarchivereloaded/

or

https://addons.mozilla.org/en-US/thunderbird/addon/awesome-auto-archive/

Excellent Matt! I was not aware of those add-ons.