It isn't that the niche is too small per se, it's that there is an existing if incomplete solution that makes the margins unattractive for the given market size

It isn't that the niche is too small per se, it's that there is an existing if incomplete solution that makes the margins unattractive for the given market size. Consider purchasing a lower-end mobile phone that is unlocked & rootable & known to run open OS options. Perhaps the Moto G4? Or if you don't mind devices that are no longer supported by the manufacturer, The Nextbit Robin.