wow, so… jackpot???
*
!**/
!*.prproj
!*.sesx
!*.aep
!*.prodset
!*.ai
!*.psd
!*.psb
!*.txt
*[aA]uto-[sS]ave*
*Audio Previews*
Found by dumb luck mixed with an inability to stop trying. I was messing with ** and as long as you have it listed as the literal path itself (and NOT as a pattern variable to an actual path), you can then list the file types you want included and it will drill down into all subfolders and grab each specified file type for backup.
The above example is my current working test file. It ignores everything except the listed file types, and it will still respect specific exclude patterns (eg. *[aA]uto-[sS]ave*)
Hope this helps another “me” avoid having to go through all that testing to figure it out!