As for mihawk and shanks its rather simple
there is no universe where i believe shanks beats mihawk in a duel
i envision a relationship similar to whitebeard et roger in the best case for shanks
where they can duel for days with no decisive winner
However if there has to be a decider , mihawk wins here .
This is what the story has implied time and time again
There isnt some niche version of swordmanship only coc users use that swordsmen like zoro or mihawk dont . They fight functionally similar because haki at its base isnt a diverse powersystem. its very fixed.
what you observe in shanks and mihawk is more of a difference in political involvement as opposed to solitary confinement. one lends his strength to a bigger purpose , infiltrating his home, gathering territory in the new world, hes the promised child, roger's adopted child really. The other even in the supposed yonko crew he factually is supposed to lead has no interest in doing so, has no grand goals for the world at large and is focused on creating a local utopia.
you see this roger wb
xebec harald too
they are all the same.