> I've often wondered how (in)expensive you could make something with a better built-in screen, the ability to play VP9/AV1 video and opus samples and Bluetooth to connect to mouse / keyboard and PC as a net and storage device.

I’ve been doing this, and the answer if you have a 3d printer is $10 US(pi zero 2 w) + the cost of whatever display you want to use. This won’t do AV1 but it’s good enough for my use cases. I’m slightly cheating on the cost as I have plethora of LiPo batteries in reserve.