Is it possible to see refrence to your characters? To see the design and style? It would make it easier for me to know what exactly to address.
from what you said some points i am confused about in how similar dl your characters look, and if they look different from each other they look boring? I personally find when characters look the same it's boring. Plus as a reader when i struggle to figure out which character is which i kinda tune out.
But for the style. You don't have to give up your style to diversify your characters. Diversity in characters can go from simple factors like weight,race, sex and other physical denominators. Or non physical denominator like sexuality. While these things change the character they shouldn't change the style.
My recommendation is refrence. Look at people, street fashion blogs are the best for that. (And you get to see col cloths bonus) study how they are diverse as well as how they are attractive OR just use them as inspiration.
Look at shows likr steven universe, the show is massively diverse in body types and skin tone while not giving up the style and the characters are interesting.
And anime wise look at full Metal alchemist. There is alot of different builds and characters that fit in that one