Sorry to interrupt fruitful discussion with beginner question.
To be happy I need just one thing. To map keys outside of hands home position onto reachable keys. Mapping cursor navigation in extend layer is great. (actually I'd propose one change in extend layer -- switching caps lock; mapping it to esc makes it badly reachable, why not to use caps-backspace instead to controll it?) But I would like to map keypad on main part of keyboard also. But their placement on traditional positions is *slow*. I would like to type number with one hand, ideally right which is accustomed to write them on numpad. I can map them on alt-gr modifier, but that kills a lot of better symbols. Is there some way how to implement numpad layer -- for example some "sticky dead key"; the dead key which remain pressed until let's say esc key enabling "uiojklm,." to type "789456123"? Well the zero is problematic, since it's in place of spacebar. Do you know better solution how to map numpad as it is? (note: solution should work in xkb also).