Use whatever I have for now, with less and less apps. Turn the device off when I am not using it. Start experimenting with project boards with modem cards.

OG iPad with a Bluetooth headset would have been a goto mobile device for most things in size, battery life, cellular reception. Never happened, of course, when they could sell you 30 devices kneecapped in various ways instead. So if what I end up building is a bit large, that is just fine.