Have a 2015 6, hopefully I can clear some things up.
1. AA/Carplay ‘upgrade’ cost me I think about £200/£250. This involves replacing the USB module in the central storage compartment, and upgrading CMU firmware to the right level. Has to be done right otherwise you can brick the CMU. It’s totally doable yourself if you can source the part (I can grab the P/N off my module if you need). It’s very worth it. WAY better than using your phone directly.
2. Nav SD card is locked to the CMU. If you can source a CMU and nav card from a breakers then it’s a simple swap over, remove your CMU and go.
My CMU died a year or so ago, I had nav previously, paid for the CarPlay upgrade (subsequently never used nav). Got a new CMU off eBay, got an original USB module, so I could perform the upgrades required to get the CarPlay module working again, but the nav doesn’t work anymore cos the SD card I have doesn’t work with the new CMU. I assume Mazda could sort but I really don’t care.