This is the most cost-effective mini PC right now, that I've found. Also, one of the smallest.

https://www.aliexpress.com/item/1005005575993915.html

I'm not so fond of it because it has a fan. But if you could use it at home, and then had a "phone conversion housing" you could attach it to a belt and have a smartphone. Run wired earbuds out it. Have a trackpoint nub.

Here is a $15 screen. https://medium.com/@lee.harding/building-a-real-time-hn-disp...

There's something elegant about only requiring 1 computing device for everything. Even put it in the car!

It's what Steve Jobs would want.

The power draw looks like it's at least 4W with a max of maybe 45W. That's maybe 7 hr with a 10000 mAh battery assuming it's sleeping the entire time and not really doing anything. Not very practical for people used to a small phone lasting all day without a charge.