I buy old kindles for this, ~50$ per piece.
they have a linux os & battery included.
you get in through a serial port and a password generated for your serial number.
downside is that you fight the kindle os in certain aspects.
I wonder what it would take to build your own firmware image if you were reusing for exclusive alternative use? Presumably there is a Linux kernel tree somewhere and you could pull the waveforms from the Amazon image?
Tried that, but when I was at the process of soldering the cables to the debug ports, the "solder spots" came out of the board :(
Now I need to press hard with a needles to make the serial port work - so it is quite hard to setup - at least until I install sshd there and setup wifi to make it work wirelessly.
see for example: https://github.com/Neelakurinji123/kindle-weather-display