yes, I've been thinking about this too, and I've talked to other people who have had the same fever dream. the children yearn for the dumb terminal.
this isn't a hardware product (people here have mentioned an e-ink tablet that can sideload, that plus a keyboard plus a battery, or ESP32-S3 etc) but rather a software product.
and that software is the glorious resurrection of vt320/vt330 buffered/paged terminals. because that's what's going to minimize your terminal's power consumption. you need to brutally minimize the number of bytes you transmit, which means having relatively small working set for any assist features (eg things like debugging might be nicer with a local variable cache). A large persistent eMMC or flash storage (but optimized for doing some decent writes) might be nice for those things. 32GB or 64GB goes a long way in a dumb-terminal context, you can use that like a portion of your RAM load if you code carefully.
btle can do some active data transmission in realtime and it has the basic "beacon, I'm still here" etc, and you can use the notification-push thing for server-side async ops. And ideally you'd use your phone as a BTLE client that routes packets/data or provides your server-side api. This can be done even on apple with the Bluetooth Browser thingy, your self-hosted website can interact with your BT device and that'll be your relay.
https://en.wikipedia.org/wiki/Bluetooth_Low_Energy#Applicati...
in an interactive scenario with an IDE-ish thing and a larger working set maybe you use more power but if you're running a nano/pico clone onboard that's dirt simple and can be done with simple buffer flush etc. And most of the ESP32-S3s come with 8MB of external SRAM.
The other major power draw is the USB, I think you could bring that down a lot with an actual hardware circuit wired right to the ESP32.
Again like, the e-ink tablet with an external KB is simple and lets you get tinkering with the editing experience in that kind of form factor! but I also think you can cut power way below that if you use something like the ESP32-S3 and BTLE with aggressive onboard caching. Don't forget to minimize power in idledown mode, and idledown as aggressively as possible. Running the keyboard as an interrupt-driven thing could cut power a lot if you can idle down between keypresses.