That was
@Berwhale ..The workaround to the 30min timeout is:
Enable Developer Options
Enable setting ' Always keep screen on while charging' (may not have exact wording)
Keep charger always connected
The LG Rebel let's you 'Never turn off screen' in the regular Display settings. It runs Amazon Music HD just fine, IMO. The one disadvantage I see is that may or may not be important to you is - you can't Amazon Music HD Cast to it from another device (Fire or not) since the LG is not recognized as a fully Alexa registered/enabled device. It will not show as a 'castable' device in the choices drop-down cast menu..
The F7 is recognized in the cast drop-down, so you can cast to it remotely. (Say the F7 is sitting next to a stereo amp with CD/line in and connected to it with 3.5mm to RCA cable). However,
@Berwhale found that doing this limits playback on the F7 to 16/44.1 (HD, not UHD)