I just received Qty 8 Refurb HD 10's (2019 models). I opened the first one up knowing about the recent developments.
These are the Basic steps I took. I did check and OS was at 7.3.1.9.
1. Don't connect to Wifi, cancel setup
2. Open Settings, device options, about fire tablet, tap serial number repeatedly to enable developer options
3. turn ON developer options and USB debugging
4. Run Fire Toolbox, update to latest if necessary
5. From the toolbox, Disable Amazon apps, I prefer manual here because I leave the Amazon Keyboard
6. Side load a launcher, I use Nova Prime, you can use the one that comes with toolbox
7. Install Google Services
8. Connect to Wifi
9. Wait 10 to 15 minutes for google services to update, don't know if this is necessary
10. Login into google
Anything after this would be your choice. I also always disable the lock screen and ads.
For my purposes, I did NOT login to Amazon. I don't use any Amazon apps at all. I realize many will need to login to Amazon to link to their own account. Don't know if this will prevent OTA updates in the future
When I first turned it on, I did check the OS version and it was at 7.3.1.19, Now, I notice and this may be normal, when I go into settings, device options, system updates, it does nothing. It kicks me back out to the main settings screen but I can pretty much confirm it is not updating the OS or my home screen which I customized.
I restarted it once, the changes I have made remain intact. Time will tell.
Hope this helps someone.