I imagine you’d spend a lot of time replicating stuff Android already does well. Like battery management, cell connection, mobile UI, touch event handling, so on and so forth.

I’d almost be inclined to counter with the opposite: what would be gained by going for just Linux over tailoring Android?

It wouldn't be too hard. You could even build an electron app to save time and money. The effort would be in making sure that the boot animation appears properly without any debugging text and that the electron app is the first thing that the user sees.

Mobile devs are easier to hire.