To add to this, I prefer Ubuntu MATE, where "MATE" refers to the desktop environment: it's exactly what it needs to be, light and responsive and useful, without the need for a GPU just to render you friggin' desktop. It's neat.
I just want to say that OLD Mac applications were exactly that, too -- even the OS was basically just "Finder" and "System", that was it. Dragging those two files to a different floppy (and swapping them 12 times to complete the copy) gave you a bootable disk. Meanwhile Windows apps were hundreds of files ... then eventually Mac apps went the same way, although not as wildly.
>contemporary so-called casual games, which for reasons I can't explain, often give me unpleasant mental sensations.
Contemporary as in "of that era" or "current times"?
If the latter, I'd have to argue this is because modern games have become very, very good at being blatantly addictive and shamelessly peddling in-game purchases that it becomes obvious the purpose of the game is not entertainment, but to engage your eyeballs.
I've successfully bought a home by self-GPG-signing and emailing documents (even in a country that has an official (albeit horrible) 'digital signature' solution).
Buy any Brother printer and use the generic Linux driver. Done.
Well, perhaps not the "replaceable parts", but that is doable with a 3D printer. I doubt Brother would come after you for printing a new output tray hinge or custom-colour button set ... and I doubt any other parts are likely to outright 'break' (my laser printers of various brands have lasted around two decades each). Any other parts, such as the image drum, would require too specialized fabrication to be done at home or at the local fab lab.
Speaking as a European, do USians have access to HÅG chairs, or some of the other brands designed by Peter Opsvik? They definitely allow varied sitting positions and have a very high ergonomic track record.
As others have noted, it doesn't look so fine on mobile, there are a ton of typo's, and the discoverability of some things is lacking -- this latter point is, I think, a really great feature, so please don't make the compressed files self-extracting.
One thing, the UI is pretty plain. I know that's the style you're going for, but let me suggest / invite you to rip off the theme from my web site: https://jan.g-b.dk/
At least, I would love it if there were. Personally, my favourite device was a Palm, my brother's was a Psion, but the basic idea is the same.
How often do you hear people complain or worry about their battery level? All the time. How often do you hear people complain that their 5-inch screen isn't 4K Ultra Retina RGBA OLED goodness? The race to thinnest form factor has always confused me; what is really the use of having a razor in your pocket, rather than twice the mAh? The race to thinnest bezel has always confused me, too; what is really the use of a device you can't hardly touch without simultaneously operating it?
I think it would be very, very welcome to have a device that did the "standard PDA thing" extremely well (unlike modern smartphone OS'es), had "just enough" features, and oodles of battery life. Just the fact that there are modern crowd-funded initiatives to make "something like the Psion Series 5" shows that there is indeed interest (if not enough to wake up Samsung and its ilk).
Although you already have a number of answers, still I want to provide another. I've been using Dvorak* for over a decade now, and I love it.
Mind you, I never claimed to be a particularly fast typist on any layout, although I have been touch typing for several decades, and on that note I am just as productive on Dvorak as on qwerty -- but at far lesser expense. That is to say, don't switch for the (mythical?) increased typing speed, switch for the ~30%(!) reduction in finger movement and pleasant "rolling" motions you gain.
True, it's not easy to share computers, be it you using somebody else's machine or somebody else using your machine. The latter can easily be addressed by quick layout changes (again, a pain to get right on Windows but a breeze on Linux), while I find that the former doesn't really bother me so much -- curiously, when I can't touch type (such as on my phone), I am hopelessly slow on Dvorak but quite efficient hunting-and-pecking on qwerty (possibly because I retain training from before touch typing?). The most glaring downside is the situations where keys and shortcuts are clearly laid out for qwerty and won't allow you to remap them (case in point: WASD in games); in reality most shortcuts are easy enough to use regardless of layout (side note: I've always been using Ctrl/Shift+Insert/Delete for cut/copy/paste, so the locations of X, C, and V don't affect me -- but modern keyboards with misplaced or missing Insert keys do).
To answer you question: I very specifically switched during the week before New Year's eve, when I could be reasonably sure that things at the office were slow enough to allow me to struggle through an unaccustomed layout. Honestly, the first days were atrocious, but within a week (so ~35 hrs) I had internalized the layout to a reasonable degree, and within a very few weeks (3? I don't really remember) I was up to my usual typing speed.
* Specifically, the Norwegian variant of Dvorak, because I need some localized keys. This has been a challenge, but a surmountable one, back on Windows (along with changing the keyboard layout for the Windows login screen, which truly defined the pinnacle of obscurity) -- on Linux it is dead simple to set up and just works.