With Misty you could say writers never explored much on her goals thats true, but they did pretty good job at developing her in personality sense which got carried over onto her story. I feel she definitely changed the most as character right after May having believable and memorable growth while bringing lot of comedy, fun interaction and conflict in main cast ,helping Ash in growth by serving as motivator and someone who push him forward, often heped in resolution of plots and saving people lives or towns from destruction through heroic acts bringing indentity and appeal toward this show(more than other girls imo). In nutshell she grew from short tempered, frustrated girl into more mature, filled with confidence person who got out of sister shadow, started to take more initiative herself, discovered value of fruiendship appreciating Ash and Brock friendship to point of being to her "new family" and overcome some fears like Gyarados while remaining playful and spunky in more mature light. In that aspect i appreciated writers work puttong lot of thouight in conveying this in captivating and fun manner.
Compared to Iris, Cilan and most sidekicks and in some aspects even Dawn which never really changed much as person growing mostly in goal sense, i find Misty went through more believable growth containing more vulnerability and hard work when episodes focused on her growth and trials she had to overcome. And as result of that writers portrayed her change as person and trainer in better, conceivable manner.
In comparison issue i have with Iris is lack of believable growth staying more or less same for most part not going through any real struggles, being followed with very little growth as far as Emolga , Excadrill and especially Axew goes.
And Cilan remained stagnant not changing much at all.
As far as Brock goes while his role wasn't big and direct like we got used to(or better term would be spoiled), he still had pretty good treatment as supporting character providing humor and fun interactions, being of big importance for Ash growth as trainer teaching him basics, preparing strategies and preventing him from doing reckless mistakes being valuable asset to group through his wisdom role of older brother he played while getting some decent development himself.
We can see that in finding common language with dis functional family becoming more compassionate and full of understanding compared to early days.
During time he traveled with Ash he transformed from at times impulsive guy which had to take care of younger brother and sisters want in more level headed and wise person learning many new things about pokemon, enriching his knowledge as breeder by learning to prepare food, potions, settle down conflict between wild enraged pokemon and about new ways of healing and taking care of pokemon eventually coming to realization what he wants to do with his life, slowly starting to lean toward doctor calling deciding to become one.
He definitely got better treatment than Max, Tracey and even Cilan whwen taking in account whole run.