My own order of the new Fire TV remote with app buttons has, unfortunately, been delayed so I haven't been able to test things out myself yet, but several of you have already told me that my Remapper app does work to remap the app buttons on the new remote.
🔥REMAP YOUR FIRESTICK BUTTONS TO ANY APP
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How Remap this buttons DirecTv and Peacook :( : r/firetvstick
How to Remap App Buttons on Fire TV Remotes