While I agree with others pointing out the oddity of a portable server in general: this sounds like a great use-case for a laptop.

Built-in battery, wifi you can broadcast out as a hotspot, and it even has a display/keyboard/mouse for troubleshooting!

An older laptop with the optical drive stripped out could have a 2.5” 5TB HDD in addition to the boot drive for some decent mobile storage.

I think the mobile app developers should run their own instances and default to that so it feels less confusing to new users.