In my head, Nina and Wylan have crisp educated accents, Inej and Jesper retain their Suli/Zemeni accents and Kaz sounds...I guess the closest equivalent would be Brooklyn/the Bronx. Very rough and low in the throat, capable of being VERY fast when it suits

Inej yes. Jesper is a little more complicated because he would have spoken predominantly Kerch at home since his father didn’t speak Zemeni. I am NOT into Kaz sounding like a New Yorker, but your head canons are your own ;)