I have to say it’s strange that AA works but you can’t playback music from USB.
This is just my opinion, but before spending that kind of money on a “new” APIM I would try checking the cables are correctly seated and if that does not work changing the USB HUB.
Why?, well it’s a lot cheaper than a new APIM. Besides, should you ever upgrade to 3.4 you “will have” to upgrade your HUB either way to get rid of the “HUB not supported” message that will most likely appear every time you start your car (this is the message that maybe you are currently having)… That’s only solved by changing the HUB. You can read more about that here → Media Hub, CarPlay Compatible - 2015-2020 Ford/Lincoln - SYNC 3
And if that does not work, you can always change the APIM… But somehow I do not really fell the APIM is the issue here, but of course, I’m only guessing.
Firmware should not fix anything, since firmware is needed for features like calm screen… not to perform a basic task like playing music from USB…
About the USB you are using, were they ever recognized by Sync3 before all of this?. Are they standard USB?.