Received my navigation SD card for just shy of $100. Grabbed it on eBay, and it appears to be the latest official OEM release: BHP1-66-EZ1H (I couldn't find anything with a letter higher than "H" at the end, and that's what the official sites are selling right now).
I ensured that the listing had a proper return policy in case something selling for ~25% of MSRP wasn't genuine, but the listing claimed it's the real deal.
Plugged the SD card in, turned on accessory mode (didn't turn the engine on), and the map worked like a charm!
I downloaded the official software from Mazda and it recognizes it as an official SD card. I get North America map updates through 03/15/2021, and it's installing the latest maps right now.
My nervosa about having that big Navigation button not doing anything is solved, and at far less than the dealership would have charged me. Huzzah!