Absolutely. 8-Pen was this for Android. (example video here: https://www.youtube.com/watch?v=q3OuCR0EpGo)
It was absolutely, hands down without question, the most accurate way to quickly enter text into a phone using only a screen. Seriously - I could type at about 25-35 wpm on my phone screen at 100% accuracy, one handed.
They'd done a ton of work to place letters based on frequency and likely combinations to make typing smooth and relatively fast.
No fucking disaster of auto-complete that I have to click to get the word I want, or when you actually have a vocabulary larger than 150 words, but auto-complete is fucking dumb and always picks the most basic option from the path, or when the word you want isn't in the phone dictionary, no hiccups if you want to misssspell something for impact. It was genuinely just the best keyboard I have EVER used on a phone.
The downside? It takes about 3 weeks of practice to actually learn, and most people just refuse to try for longer than 10 seconds.
I still find it incredibly frustrating that qwerty, of all god-damned things, is the input formula we've picked for phones. It's fucking dumb.
As for the layout, the previous Reform has free keyboard firmware so you can change the keymap however you like. I assume this one will be the same.
Personally I don't think there's a good traditional (as in, flat and square mesh of keys) keyboard in those size ranges. For me it's regular keyboard if there's space, gamepad-like at around the 7 inch size, then BlackBerry style if any smaller.
As an emacs user, I can confirm this is absolutely the case except on the bottom edges of an elevated keyboard, where you can palm press which is the most comfortable. This leaves out every traditional portable device's keyboard, but keys specifically designed to be palm-pressed on devices like this would be awesome if you could only find a way so it doesn't get pressed accidentally.
https://www.aliexpress.com/item/1005004014867323.html
Pinephone keyboard has extra GPIO exposed on PCB (on my request, because I had this mod on mind, early on) so that this is doable, and I made sure the firmware is safely flashable in field: https://xnux.eu/pinephone-keyboard/ so that it can be extended to support this.
That would add 6 different or 2x3 symmetric modifier keys to the bottom (3 for each hand) in addition to what's on top, and would make the keyboard much more usable for thumb typing.
Why waste 2-4 fingers that are usually resting on the bottom?
I have to find some time to attempt the mod. :)
Do you have a Mozilla reference for this design principle? I've always wondered about this difficulty, but thought it was just an accident.
If you have only two thumbs available for typing, that can only reach about half the keyboard each (quite a hard limit!), you realize how asumptions this layout makes suddenly don't hold, and normal things like ctrl+a or alt+shift+1, or alt+shift+any arrow are almost impossible.
Yes, you can modify the shortcuts in i3wm,screen,... (not Firefox, which is quite resistant to modifying keyboard shortcuts by design) specially for your "PDA". But if you just want a portable device you can painlessly migrate your tasks to for a few days a year, spending a lot of time designing shortcuts for your DE, or terminal multiplexer, is not that appealing.
It would be probably more useful if key layout was designed from the start to allow various modifier combos to be invoked by dedicated keys, or layout designed so that more modifiers were symetricaly placed (or something even less traditional, and have all letter/number keys on the right and all modifier/modifier combos/special keys on the left), instead of wasting space on big enter key, or big spacebar (for which you may spend several HW iterations just to make those right and reactive on their whole surface), etc. Due to size, it's usually uncomfortable to press 3 key combos even with the keyboard laying on the table.
Also comfortable position for the most comonly used keys (usually modifiers) is not on the edge of the keyboard. That's really pretty bad for your thumbs.
The layout of these things just needs a lot more thought.
Sad thing is, that if someone would spend a ton of research and thinking on making something usable and comfortable, most people would completely poopoo it as a freaky unusable layout, just because it would look unfamiliar, lol.