Iris in the masters 8 was just oddly handled in general. She lost without using any of the special power ups that all the other champions used. If you’re a young trainer who just became champion, you should be prepared to use mega evolution, z moves, or dynamax. I don’t think she should’ve beaten Cynthia, because I really wanted to see Ash vs. Cynthia and not Ash vs. Iris. Iris losing to Cynthia sets up Cynthia as powerful and gives Ash an extra motive to win against her. Still, compared to the other trainers in the tournament, Iris seemed out of her league. I think we should’ve gotten a flashback episode showing her evolving her Pokemon, becoming champion, and mastering one of the power up techniques, to give her presence there a little more meaning. It seemed like she had just suddenly transformed from the anime character to the character from the end of BW2 games.
I don't know if Iris using one of the battle mechanics would have necessarily made her role in Journeys better. Her team is mainly fifth generation Pokemon, so they wouldn't have Mega Evolutions available. I don't know if Z-Moves would have fit with her, especially when I think Ash was the only trainer in the Master Class who did have access to that mechanic. Dynamax could have possibly worked, especially when Cynthia used it. Considering that she and Ash are the youngest Champions in the tournament, it does make sense in a way that she's out of her league.
Considering the last time we saw Iris prior to Journeys was chasing after Rayquaza in Johto, her characterization into a Unova regional Champion really does feel off. A flashback might have helped, but I don't think it would have changed much for me, especially when it's a lot of off-screen development for the audience to buy into. You can make a good argument that her Champion status doesn't have much weight in a sense when she only loses. It makes sense that she'd lose to both Ash and Cynthia, but when she only loses, it can make her Champion title feel more hollow.
Iris not using a gimmick is good, since it shows her strong without one. Although the real issue is Gamefreak never gave Dragonite a Mega Evolution or something similar, so it's the same reason Ash's Dragonite barely had any development because they couldn't give it a gimmick like how his Gengar got to G-max, or Mega Lucario, etc. That's more on the game side of things, same reason AshGreninja exists because the XY games never gave the Kalos starters Mega forms for some reason.
The issue with Ash's Dragonite was less that it didn't have a Mega Evolution, but that they just gave him a fully evolved Pokemon, so there was less for them to do with it in general. Gengar had its Gigantamax form and Lucario had Mega Evolution, but aside from maybe Lucario, Ash's Journeys team didn't particularly have a lot of development to begin with. To be fair, this was an issue with Journeys in general, not just with Ash's team. Unless a Pokemon was being caught, getting an evolution or some kind of upgrade, most Pokemon tend to barely get much attention in this series. While it's far from his worst team, this kind of made Ash's Journeys team feel more hollow to me, which is insane considering how long I wanted Ash to get a Lucario.
As for Hoenn not having a rival, Drew was basically the replacement for Gary in the series even if he was for May, they even had Ash and Drew battle each other too. Drew did start off as "Contest Gary" although he develops into his own character as it goes on. And as said, Ash not having a real rival didn't matter too much, although I do wish Morrison or Katie were introduced prior to the league or earlier so we could get to know them. They would have been fun to have around.
Morrison was okay, but I thought that he worked fine as a League rival. Katie being introduced sooner would have been cool though. It's a shame that they didn't show a lot of female trainers in Leagues or give Ash more female rivals. Technically, I guess Bianca would be it, but they butchered both her characer and storyline from the games so badly in BW. She was basically a less competent and more annoying version of Barry.
But overall these are minor quibbles. Most of the complaints about the anime started heavily due to Ash's characterisation change between sagas from BW onward. Looking back the whole OS/AG/DP era was really interconnected in terms of plotlines, stories and continuity, that I feel we took it for granted when it was airing. Sure it was never perfect, like the writers ignored Pidgeot for 20 years, the Battle Frontier arc barely had any memorable Kanto locations revisited, old characters still wound up forgotten, etc....but it still had a stronger sense of being all the same series back then.
No, I'm pretty sure that people complained about the anime before BW. As much as I love DP, the Sinnoh League was the first controversial defeat for Ash. People to this day still complain about Tobias, wishing that Ash could defeat him now as if that would mean anything to anyone besides those still bitter about that battle after all these years I'm sure that people wanted Ash to be replaced well before BW too. Some fans weren't a fan of the pacing, especially the year gap between Ash's seventh and eighth badges, even though a lot of huge developments happened during that time. Not to mention the backlash towards Misty's departure. The notion that the complaints were just minor quibbles prior to BW really isn't true.
OS/AG/DP were three different series. You can argue that they felt more well connected compared to later series, but they were still largely stand alone series. You didn't need to see OS to watch DP.