The Psion Series 7 was a netbook-sized machine with a VGA screen. Came along just too soon to have wifi and bluetooth built-in, just too late to be competitive with the ever-shrinking notebooks like the Toshiba Libretto range.
The nearest thing to psion devices today are made by [Planet Coputers](https://www.www3.planetcom.co.uk/); their keyboards are designed by Martin Riddiford (the designer of the Psion Series 3 and 5 machines), and with fast charging over USB-C they're as close to what you're asking for as you'll find today.
I think the hardware is great, but I definitely don't want Android or Linux and that screen looks power hungry. A similar form factor running a lighter OS (like EPOC) with an eink or other low power display would be pretty cool but also would likely only have a market of one.
I used a Psion 5 back in the day, and read ebooks on it which gave the battery life a torture test. A pair of Duracell AA cells would last 20 hours without the backlight, and about 7 hours with it.
A Gemini or Cosmo with the wireless hardware turned off and the backlight at about 30% will handily exceed the 7 hours, but not reach 20 hours. Except you can top them up with any USB-C booster battery.
AAs and AAAs are an environmentally wasteful solution: they were great back in the day when laptops sometimes had a mere 60 minute battery life and required a kilogram-sized brick to charge from the mains, but those days are long past.
Your Psion 5 battery numbers sound about right. In comparison, a new Macbook Air can play 18 hours of video on a single charge. Thirty-five years ago, the Radio Shack Model 100 could run for that long on a set of 4 AAs. It should be possible to make an ultra-low power device that can run the Psion 5 apps (ie simple apps) that can go for weeks on a single set of batteries or charge.
I really miss the Psion 5 / Radio Shack Model 100 / eMate class of devices. I think they are still better in some respects than modern Android-phone-with-a-keyboard devices.
The Psion 7 (& Netbook) have IMHO the absolute best keyboards for their form factor. I still use my Netbook for distraction-free writing. It doesn't run on AA batteries but will easily last a couple of days on one charge. At just over 1kg it's effortless to carry about & the shape makes it easy to clutch in one hand. For me it's the ideal note-taking machine.
The nearest thing to psion devices today are made by [Planet Coputers](https://www.www3.planetcom.co.uk/); their keyboards are designed by Martin Riddiford (the designer of the Psion Series 3 and 5 machines), and with fast charging over USB-C they're as close to what you're asking for as you'll find today.