I have recently taken to gathering old phones and putting Nerves on them (Nerves packages up the Erlang VM into a mainline kernel to run on a variety of devices). I got inspired by a talk by Marc Lainez at Goatmire who had put it on a variety of exotic devices (e.g., a Wii).

I am by no means an expert at this, but starting of with a device on the Postmarket OS list and Claude, you can get a lot of these old devices working with mainline Linux and nearly all drivers. The newer devices are a bit harder, but if you go back nearly a decade, they're very easy to get working.

So far I have repurposed a Fairphone 3, Nexus 5, Galaxy S20, and Galaxy Tab 3.0 7". All of them (except the s20) with all drivers working compared to stock. Others have done a lot too: https://github.com/mlainez/awesome-nerves-systems

For DIY projects you can use host USB, or get one of the Adafruit FT232H type boards for GPIO pins.