This is the closest thing I found just now by googling, though by similarly 'directional' reasoning I think Del and Backspace don't belong in the central column and should be on opposite sides: https://www.keyboardco.com/keyboard/truly-ergonomic-227-mech...
As well as enter/backspace/delete, it's also useful to have esc/tab near spacebar. https://github.com/manna-harbour/miryoku
e.g. A popular off-the-shelf keyboard which supports this layout https://www.zsa.io/moonlander/
> ...below the space bar
Well. Then you'd need to move your hands, or stretch your fingers a bit. It's better to have a smaller spacebar key, to allow for extra keys.
Here's the XCompose file I use:
What you're looking for, is a ISO Level 3 Shift (at least as far as X11 is concerned. Don't know the terms on windows).
I configured my keyboard to use the WIN keys as that. Super I only use for window-management related things, which I don't use is all the time and I can configure the actions to be on my right-hand. Therefore, since CAPS is useless, I remapped it to Super.
This way, with the dead keys, you can create many composed symbols. For writing French, an US layout is perfect, and I can type as fast as on a French keyboard. For your particular case, I don't know how to create the Ż, but Lvl3 + e + Z -> Ź. It's likely that if you used a different layout, this would be possible.
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[0] I've tried multiple times to get a keyboard layout like on a mac, with its deadkeys system, but never managed to remap the AltGr properly.