[stumped]

Actually , after more reading and sorting through the the middle. Os 5 had elements of both lolipop and marshmallow, .e. 5 and 6. Os 6 is sort of based on nougat, I?e? Android 7 despite being labelled as os6.
Amazon have pushed 6.3 as software update tom2014 through 2017 models. The 2018 model should just be tweaked for Alexa, with a software update promised to bring that to older models.
Confusing as that is, my main points were correct. You have to find Google apps that are compatible, and you have to install in the right order. I always do account manager first. Then reboot. And the later link that I posted says to use a version 5 account manager for os 6 as later versions do not work. As the OP found, But that is contradicted by the xda article.
So we are all the way back to: If you want Google on fire tablet you have to be prepared to read various contradictory advices, or watch contradictory you tube videos, then boldy go for it. Back out, try again....

There will never be a magic pill perfect for dummies how to guide.

I also read the stuff about the SD card option whereby it mounts as internal storage, but that should present as a user choice.

That really is it from me. There is nothing more I can say to simplify things.
Do your own research and YMMV.

My tablets are working except I never did get Gmail to work. I used Amazon's email app instead. I gave up with fire TV, and with old version OS4 or older Kindle fire ablets.they would have to be rooted.

OS5, 6 should be doable, it's anyone's guess whether it will stay like that for future fire tablets.

Arguably it is better to buy android and work on workarounds for streaming Amazon video in hd on those. I have one for that. PM if interested. With that sorted. My Lenovo tablet does everything that my Amazon fire tablet can do, and a few things it can't.
The main attraction of fire tablets is the price. But you get what you pay for....