I actually found a solution. Note that I posted that background operation worked with two different Firestick 4k's but would not work on my 4k max. I found that if I disabled Ambient Experience on Firestick 4k Max, Pandora would stay running after I pressed Home and start project M. What would be a nice feature for Pandora would be to integrate project M into the Pandora App so folks aren't stuck with the static Pandora screen. Even if a user shuts off Ambient Experience, the projectM app as a separate app will not actually respond to any music unless you send a specific ADB permission command. This is not something most Pandora users would want to do or be able to do.
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