Update in case anyone cares:
So I messed around with my music collection and here's what I've discovered:
1) there appears to be a 20,000 songs hard limit on Uconnect, at least there is on my 2019 Durango R/T with the upgraded HK audio. No matter how I break up my collection it will NOT see more than 20,000.
2) some music files have unusual characters in their metadata. When Uconnect encounters them, it ignores the file. We're talking about things like slashes, foreign characters and accent marks, asterisks, things used in scripts and if not escaped properly within the script can cause it to fail...cause whoever wrote the Uconnect software did NOT escape special characters properly.
3) some music filenames have characters that Uconnect doesn't like - for the same reason as #2. Newer versions of Windows and MacOS are very forgiving about filenames. Other OSes are not.
4) the fewer directories you have the faster Unconnect scans.
5) USB3 and fast flash makes things MUCH faster. (I used a very fast microSD card and a USB3 card reader - it was much faster than slower flash devices and regular USB).
If you have a Windows machine you can use mp3tag, a very powerful and free tag/file editor. I was able to load my entire collection into mp3tag at the same time, identify all the "bad" characters and create scripts to apply fixes to the entire collection.
general process:
I made a copy of my music collection to the USB device.
I used mp3tag to copy the "album artist" to the "artist" field in the metadata tag because Uconnect uses the "artist" field to sort.
I then used mp3tag to rename all my music using artist-album-year-tracknumber-trackname format and move them to a single directory.
After this process Uconnect was able to see 20,000 songs. I had to cut my collection down below 20,000 songs.
Things I haven't tried but might be worth a shot: use a USB hub to put music on 2 flash drives - probably won't help.
Placing all files in a single directory got the scan time for 20,000 songs to about 30-45 seconds. The scan only happens if I remove the USB device, otherwise it's ready to play as soon as I turn on the HU.