"degree of mobility n portability with self custody + secure" with hardware price as key factor can be easily solved by buying dirt cheap new/used HEADLESS minipc minimal RAM CPU X64 - full ubuntu / other linux - hence wasting time on andriod is useless if not work in one shot try - no need reinventing wheel - 12V from car or battery take it anywhere in camping or vacation or in backpack - just not in pocket - far cheaper than high RAM HDD expensive phone device 
Discussion
Oh I already tinker with small machines, especially the rockchip boards w/ gentoo. Getting relay on a device I already had laying around (and are ubiquitous) was simply a fun and interesting exercise.
If you are adventurous and your model of phone has unlocked bootloader, entirely possible to do away with Android completely and just load up a busybox based kernel!
exactly -> purpose / power / price