Since I'm into 3D printing, I know of this project: https://github.com/gaifeng8864/klipper-on-android
It allows an android device to run a printer that uses the klipper firmware.
This was going to be my answer!
my favorite use is ipod replacement.
That's what I did. 1tb microsd full of music, movies, and TV shows.
I read about some PCBs that let you repurpose old laptop screens but I'm pretty sure that's more complicated with phones and tablets. These have tiny and very specialized electronics. Oftentimes not built in a modular way. And people tend to break them, give them away or dispose of them. I don't see people repurposing these devices.
And it'd be hard to bypass the boot time and Android experience. Sometimes you can flash a custom ROM like LineageOS. Though, that's still Android. Other operating systems aren't really a thing within that ecosystem.
I like to flash LineageOS and then use extra phones/tablets as a kitchen radio or TV or as a TAN generator.
You could also install Termux and install Linux software. Like a webserver or something like that.
If you are willing to keep airplane mode on all the time, you can install Krux apk and use it as an off-line Bitcoin transaction signer
It seems like a pain in the neck because of the difficulty of writing code for Android, especially older versions, since the API's keep changing and the older SDK's might be hard to find and use (I haven't tried). It helps if you can root the phone, but that isn't always easy either. Plus, a phone-sized PC display doesn't sound that useful. For a wall or desk display, you might be better off with an Inkplate (e-reader with reflective e-paper display) or similar.
iPod replacement, dedicated smart home device dashboard, dedicated navigation/entertainment system in the car, dedicated UI for something on your home network or weather info or headlines or whatever website you want to check regularly in one place
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