While writing this comment, I discovered that there's a website selling new old stock? Can't vouch for its authenticity, but you should go for it! https://shop.pocketchip.co/collections/frontpage/products/po...
If you want a small device to play old games on via an emulator, I can recommend Pocketsprite [1]. You can even assemble one yourself.
FWIW, I'm looking for a small portable screen (1080p, IPS) to combine with my MBP 2015 (via TB2 or HDMI but I also want it to have USB-C for future compatibility).
Sadly, most of these points are mutually exclusive. Android tablets are built for the average user who would have neither use for an hackable OS nor for exposed GPIO ports. Adding them would represent an added cost with no returns with respect to the market they're into. Also, most if not all Android tablets - actually their manufacturers - are hostile to letting techies install their OS of choice, and implement tactics to discourage them, mainly keeping the hardware closed. To date, the Pinetab seems the best example of a tablet designed from the ground up to be of interest both to users and tinkerers because it encourages to experiment with open operating systems. https://www.pine64.org/pinetab/
https://hackaday.io/project/164845-dlt-one-a-damn-linux-tabl...
However, Precursor is more about evidence-based hardware security, and less about features and usability. To better understand the principles behind Precursor's design, please have a look at https://www.crowdsupply.com/sutajio-kosagi/precursor/updates....
The TL;DR is that it's a development platform for applications that strongly require secure and/or trustable hardware.
https://www.kickstarter.com/projects/745629624/cutiepi-raspb...
But if it's well-marketed to the hobbyists, and software support is adequate, then maybe.
Mid-to-high 5-figure to low 7-figure unit sales (for a 'wildly' successful device like the Raspberry Pi) is what it seems to be looking like for a successful open mobile(-ish) device. As long as the businesses offering them are sized appropriately, it's doable. Just need to be smart about re-using enclosure designs etc and have long production runs to help amortize things like regulatory certifications etc. For hacker devices, they can sell this stuff for several years between major revs and their customers appreciate the longer product/parts availability.