You're probably not going to get an internet connected device, with battery, with cameras, with user input, and one that fits in your pocket, cheaper than a used phone. I think it might be better to just create something like StartOS or Umbrel that runs on a phone. The reason mobile lightning sucks generally is that the operating systems don't "wallets" to keep running when they aren't in the foreground.

It'd be cool to have dedicated device though. There might be form factors and capabilities that are better delivered in a dedicated device than a smartphone.