Here’s the Amazon link. Yes, I installed it myself. Didn’t wanna pay $300 for labor lol
I don't believe you can downgrade the firmware. Installing the version 70 first is a requirement for the "official" android auto. I paid $162 for the USB cables and dock ( https://www.amazon.com/gp/product/B07KRPSRKH ), and $10 for the trim kit ( https://www.amazon.com/gp/product/B01L8GHB7O )
I found the instructions to follow here: https://forum.miata.net/vb/showthread.php?t=679114
There's a link to an 'hdrive' site, find the NA firmware, put them on a USB key (16gb or smaller) and follow the instructions to the letter. It's easy.
Then find the folder for the videos of the cable install, find the closest match to yours. Also easy.
Firmware took an hour (mostly sitting and waiting). Cables took an hour in my 2018 CX-5.
Both were easy as far as anything technical I've ever done. Very easy. Just take your time, follow each step carefully.