Brock's development ended with Johto. Ash didn't rely on him as a mentor anymore and he was pushed to the sidelines because Ash helped May and Dawn instead.
Please do explain how exactly? When Brock developed more in Hoenn than he ever did in Johto in learning how to become better breeder, learned about various ways of pookemon healing, preparing potion or pokemon food through course of Hoenn generation., Met several other breeders like Yuma and Isis.
Applied his knpowledge in various issues concerning pokemon like incident with Shiftree who kidnapped nurse Joy, saving Mudkip leading to capture, Solrock who caused drought in village being misunderstood by people, Donphan which fallen in love. Or curing sick pokemon like Sharpedo, Bagon, Nuzleaf, taking care of baby Bonsly growing as pokemon caretaker a lot in noticeable way.
Caught more pokemon, evolved his Lotad and Mudkip, battled far more compared to Johto. Had sparring battles with Ash like Torkoal vs Lombre or Growyle vs Mudkip. Entered various events like contest, pokemon orientering, cheerleading deceiving institute, fought at Pewter gym in ntag battle with Ash(one of his best battles imo) etc.
Along with events during Battle Frontier and taking care of suck nurse Joy starting to ignite within Brock character more and more interest in becoming doctor.
Feeling like valuable member of AG cast interacting very good with May, Max and Ash.
Likewise statement of Brck not mentoing Ash in AG is simply untrue. What about preparing strategy after losing vs gym leader Brawley? Against Winona? Advicing what strategy and attacks to use against gym leaders like Flannery, warning him about special abilities like overgrowth, hidden power and frenzy plant?
Making sure that Ash and co dont fall recklessly without proper plan in dangerous areas, eat poison food, searched for specific berries required to restore pokemon health etc.
Often providing tips and moral support.
Brock developed more in Hoen than he did in Johto, battled more and received more focus yet some weird picture is painted of his character being "apparently "sidelined more there" not having role? Ash and May obviously got more focus(main star and females always get more), however that doesnt change fact that Brock still got more spotlight focusing on his career and subtle, but still visible character growth than it was case in Johto series where he was sidelined even more than Misty did.
Who at least got some compensation in Master Quest with Whirl Islands, several episodes focusing on her growth bonding with other characters like Sakura finding common language in helping to overcome problems with older siblings, Dorian in proving her worth as trainer, revealed why she liked water pokenon and growing to understand and respect better others like hotheaded trainer of fire pokemon who tried to catch Macargo etc. While utilizing more her pokenon and played subrole in several episodes like dealing with Ninetales, helping others like girl singer to retrieve her Igglybuf, tried to stop wild Sneasel, protect Diglet village from burglars etc.Beiung more active and lively in characterization due to more exposure.
What Brock exactly got there? Not much aside from having closer bond and better chemistry with Misty contributing to original trio interactions feeling more fluid, emotional and full of flare and unbounding support compared to Ag cast imo. But in screentime Brock still got better treatment in Hoenn in reality.