[Fire Q&A, Help & Troubleshooting] Nazo:
I turned off the WiFi at the router while it was downloading the update (eg right after it started the process) and it popped up a dialog that said that my connection was slow and it would continue downloading in the background. I still had to turn the WiFi on to complete the setup process after I got that far, but I kept turning it off between steps (and in particular right after it finished the whole thing about getting account details and all.) It never updated the system. So mine is still at the 5.4.0.0 it arrived running. It wants to update, but can't seem to if I'm keeping it busy. Hopefully it won't sneak and do it later. So I guess this method at least works to get you started.

It seems there is a firmware binary for downgrading via adb sideload on the recovery. I'm not sure how safe this is (and mine is still on 5.4.0.0, so I have no reason to flash 5.4.0.0 to it.) KingRoot says a root is available, but it sticks at 1% so I guess it's not working. I'm trying the "updated tools" posted in this forum here: https://xdaforums.com/amazon-fire/help/fire-7-2017-7th-gen-downgrade-debloat-t3699937 and so far everything is saying "success." It doesn't include a root option though. It does say it blocks the OTA updates? It uses adb uninstall from the user level, so I'm not sure how effective that is. Guess it can't hurt anything anyway. Isn't 5.4.0.0 rootable? EDIT: KingRoot now no longer shows a root option. With OTA updater removed I can't get to the system section that shows the current OS version (since they don't have a proper "about" page with actual details.) Maybe it did manage to sneak that update through. (I think it was 5.4.0.1 that it was trying to do first.) EDIT2: Kept retrying KingRoot and suddenly the root option appears again. I think maybe the method it uses is unreliable or something. This time around it didn't freeze at 1% but still did take a long time there. In fact, it's very slow the whole way through (I haven't the slightest clue what it's doing. Normally rooting doesn't take very long on any device, but I guess it depends on the exploit used. It seemed to me more like this was downloading a significant amount of data rather than trying exploits though.) Didn't work though. I saw someone else say it can't be rooted from anything past 5.3.3 something? I guess you can't downgrade to that though or people would already be doing it.

(To be clear, I'm not a fan of KingRoot -- I consider it bloatware and potentially dangerous -- but it seems a lot of Amazon Fire users are using it, so I have to presume it at least is somewhat reliable for them.)

Anyway, at least with all this it has the Play Store, an optional debloat that removes Amazon apps (I may put the app store back just in case, but most of it is a bit excessive IMO,) it removes the ads, and it offers a means of changing the default launcher (sort of. It hijacks anyway. The end result is close enough if a bit messy I guess.)

BTW, as a tip from someone who's used to seeking out lower firmware systems in general, don't buy straight from the source or a "fast" place that goes through stock quickly and doesn't keep a lot at a time (since everything is so automated) like Amazon. Go to a brick and mortar store like Walmart, Target, etc etc. Even online they are more likely to have backstock that has been sitting on a shelf for an unknown length of time. There are never any guarantees, but this does increase your chances of getting something older.