You could just use an old second-hand phone for under $60. You’d get a built-in battery, much more RAM, and a screen included. For the output, just use a $3 ESP32 connected over Wi-Fi or USB. Seems like a much better deal for a lot of projects.
OF course, that shouldn't matter for most tinkerer jobs like a CarPi. But anecdotally, I tried using my old iphone 13 (idk why I tried this) and then a couple old galaxies for my car project taking data from bluetooth dongle and relaying to a custom gui I made. Of course also dealing with power cycles and screen, data trasnfer etc.
Very, very quickly it became easier to justify buying a SBC for the same project
I run a headless laptop as a server and it works great, and just sips power. It didn't have Ethernet built-in, so I just replaced the WiFi card with one and it worked just fine (and also let me panel mount the connector for a DIY 1U enclosure).
While I did not get it myself, I recently saw a business laptop I was hawking with 8GB of RAM and a relatively modern CPU sell for less than $50 on eBay. All that was wrong with it was a bad back panel; just some broken plastic which I wouldn't have been using regardless.
I could see myself running a whole rack of headless laptops in the future. It's more likely than me buying a single new Pi at these prices.