They all need more characterization. Their biggest problem is underscored very well in your post: they're relatively interchangeable.
Whatever the direction, DC needs to pick a lane and drive. Give them quirks that fans can identify and make a part of their voice. Say what you will about Tom King, he made Kite Man a thing and now that guy's adapted to acclaim on a TV show. Everyone knows "Kite Man. Hell yeah."
Any of Diana's supporting cast can use some of that. Not a meme per se, but something identifiable. When you remember Alfred, among many other things, we know he's the mother figure of the Bat Family, has a scathing dry wit and makes excellent cucumber sandwiches.
At the end of the day, Steve and Etta are just two characters. I think they are important and even if they are not always treated or developed well, that's hardly unique among the WW supporting cast, Amazons and villains. Other important characters can be added to that circle without the need for them to go away or be shoved into the background.
For me, Diana's ideal Man's World core cast would be Steve, Etta, Donna (as Wonder Girl), Ferdinand, the Kapatelis women, Mindi Mayer and a boss for Steve (either Darnell or Michaelis). They should be to Diana what the Daily Planet staff and Supergirl and Krypto are for Superman and what Dick, Alfred, Gordon and Babs are to Bruce. Everyone else comes and gos from there.
Etta Candy is way more interesting and fun than Julia and Helena. Like someone said, Etta is Lizzo or Melissa Mccarthy. She is a boastful woman of action that loves her body nearly as much as she loves eating and chaos.
I never understood the appeal of Julia. She's a nice intelligent woman, that's it.
OTOH, if something doesn't work they should be willing to change otherwise stagnation will cause it to sink. It's a matter of deciding whether a character or concept just needs a new writer or to discarded entirely in favor of new ones.
I agree with the posters above that Diana's supporting cast needs to be a combination of her Amazon family and her other found family in Man's World.
I used be "meh" on Julia, but I've grown to quite like her a lot after some of my recent Perez re-reads. Vanessa as well.
They could just be balanced out with Steve and Etta more. Perez had them as the main cast over Steve and Etta which I didn't like, but balance it out more and we'd be fine. And I'd like to see Vanessa as a friend for Donna.
Strongly agree with you, which is why I loved when Perez made Steve an old man and a totally secondary character after the first arc.
I also liked that Diana did not have a love interest until the sad arrival of Barnes in Jimenez's run
Although I must say that the approach on Steve that Rucka did in rebirth I liked a lot.
But I never felt very fond for the human characters that surround Diana (except Mindy Mayer perhaps), I prefer the Gods and mythological creatures.
Still a downgrade from the epic Marston era Etta Candy and Holliday Girls.
That is because for the most part. Her power level in WW 2017 is not too bad for an origin story where she is just starting to discover her power. W84 is a whole different situation. The writer and also director said WW would be at the highest of her power. Yet the end result was a hero thgat was depowererd for most of the movie and the action was sub par at all times. While WW 2017 had at least one really good action piece in No Man's Land.