I wouldn't suggest an Android device here for the simple reason that for the use cases mentioned Rockbox is a very poor choice.
Android offers a native solution capable of running both use cases orders of magnitude better than Rockbox will.
[Saint]
Edit: I forgot to mention the main reason why OP using an Android device for this use case would be a tragically bad idea:
Neither doom.rock or rockboy.rock are compiled for Android application builds, for the above mentioned reason.
Control is another potential issue, not all devices (practically none these days) have anywhere near the amount of physical keys (capacitive or otherwise) as some games would require to be fully playable - this is the type of thing you would really want at least a 4-way DPAD and select for at least - but it absolutely doesn't matter due to the point from the prior sentence, these plugins don't even exist on Android targets.
Apologies, I should've made that clear in my initial post.