Thank you @xhostluv, by your idea I have tried the HALauncher on a Fire TV and this can be configured optimally as a Leanback-Launcher alternative.
Since the HALauncher is not recognized as a launcher app on a Fire TV, I modified the APK accordingly. This is now defined automatically as the default launcher of Fire OS (if only the HALauncher is still available and the Amazon Launcher has been deactivated or uninstalled).
If there are multiple launchers installed (but not the Amazon Launcher), the Fire OS launcher selection dialog will appear, for choosing which one you want! Alternatively, you can also choose the HALauncher via Launcher Hijack.
Modified version of HALauncher for Fire TV
- Recognition as Home-Launcher App on Fire TVs
- Selectable via Fire OS 5 or Launcher Hijack
- Ignore Updates