Hi Sasha, here are some further thoughts.
- You might get inspiration from the post A Macro to Enter Accented Characters (Diacritics), although I haven't examined it.
- I can imagine a macro that is triggered by your chosen key and then pauses till another key is entered.
- To implement a true dead key I would be looking at a lower-level solution than KM, at least as the starting point.
a) It looks as though setting up a dead key in Karabiner-Elements is not easy.
b) QMK offers what it calls one-shot keys (same thing). QMK is for programmable keyboards only, of course (I would no longer use any other kind). - I am curious about what you particularly like about the dead key approach.
- I wonder if, instead of a sequential (dead key, then letter) approach you might consider using spare combinations of modifier keys, for instance Shift-Control-[a/e/i/o/u] to enter the desired character with one "modifers+key" movement. I think I would find that faster for regular typing.