There's currently a exploit which allows for system user access. This allows disabling core FireOS features like forced updates (which intentionally break custom launchers) and also allows setting a custom launcher. It's simple and can be done with just an Android phone (or ADB).

I've set Projectivy as my launcher and now it starts directly instead of the ad-ridden Amazon launcher.

https://xdaforums.com/t/system-user-fireos7-os8-all-fire-cub...