I provided an answer to Really minimal install (dumphone++)re - #5 by aibd on a similar theme and describing a method you might care to copy.
The build method described there might produce (for example) a Mindfulness mobile phone, with minimal distractions.
In: Mobile phone, Clock, Alarms, Music, Bluetooth for transfer of recordings to the device, Files, Contacts, Camera and Notes.
Out: No Browser no App store.