You're right about you being a bit nitpicky.
Suffice to say that Catelyn was a huge improvement, because in the show she had the ability to judge the totality of her mistakes, and she was never written as so prideful to be foolish, which allowed huge swaths of the (male) fan community to dismiss her as a vain fool. I'm talking about moments like her urging Ned Stark to go to King's Landing because it would be a great honor. Rather the show displays Catelyn's tactical brilliance, at least in terms of realpolitik constantly, as the show underscores she is always right. She tells Robb not to marry Talisa (something she wishes she could've warned in the book about Jeyne, but it's tool ate so she keeps silent), she said don't send Theon Greyjoy back to his father, and she was able to even negotiate a possible peace with Renly Baratheon.
Robb being aged up is just the necessity of the medium. Giving him more onscreen dynamism is also because of that. I understand the novelty of subverting reader expectation by making the Boy King (the ostensible hero in a fantasy story) a secondary character as told from his mother's perspective, but as the show wishes to build out and fully flesh out the Starks, he needs to have a more central role, especially for the Red Wedding "twist" to work, so developing a romance between him and Talisa instead of making it a one night stand is fine.
Margaery in the book is an Anne Boleyn clone that is, again, intentionally underdeveloped so readers must cast their own opinions and interpretations on her. The show does not have that luxury. They could've made her mostly peripheral, but instead we got an upgrade with Natalie Dormer (ironically a former Anne Boleyn) playing her as a major power-player in the series. She is actually one of the strongest elements of the show's best seasons (3 and 4), and her manipulations of Joffrey and Tommen are actually very well written television, albeit quite creepy in the latter's case. Her scenes with Olenna are even better, another improvement in the show considering the Queen of Thorns is also barely in the books and we are left merely to understand who she is by implication.
I won't go through all your points on Sansa, save I agree that it was foolish for Lititlefinger to marry her to the Boltons and that it was forced as the literary character (or the Littlefinger of the first five seasons) wouldn't have made such a ghastly mistake. However, Sansa being manipulated by Littlefinger to marry someone who will not really help her? It's heavily hinted at in ADWD and in sample chapters in TWOW that he's doing that to her and she is falling for it. I actually think Sansa has learned a lot and really isn't snobby at all in the show, not anymore. She is still very feminine and always will be, but she is doubtful of Jon because he gives her reasons to doubt him (no one thought him entreating with Daenerys in season 7 was a good idea and it only worked because the plot needed it to), and her seeking revenge on Ramsay feels like an authentic growth of the Sansa who spent years being beaten, psychologically tortured, and threatened. Keep in mind that Arya becoming an assassin and murdering people in cold blood is a far cry from the little girl of the early books who just wanted to be reunited with her mother and keep her friendships with Gendry and Hot Pie in tact. They change over time.
Again, you act like all of HBO's credits are The Sopranos, Deadwood (which I think is overrated), The Wire, and Big Little Lies. They also produced True Blood, Entourage, the second season of True Detective, and so man others. Game of Thrones is one of the very best dramas on TV, ever. It's had missteps, and may lack in the sophistication of, say, The Wire, but its narrative is so intricate and finely woven (yes because of the books) that it really is more ambitious in terms of plotting and scope than any show on television and it clearly works.
Also your generalizations ring false about "all the best moments are in the final two episodes after season 3." Not when Joffrey's murder, Tyrion's trial, the Red Viper vs. the Mountain, Tyrion meeting Daenerys, Hardhome, the death of Olenna Tyrell, and Cersei going Targaryen on the Lannisters all happened in early or midseason.