The best part of Serena's development was very subtle. In Serena's first performance, Fennekin fell and Serena went right down with her. Even Eeeve falls in her Showcase debut (It was her debut?), Serena rallies her to finish. The old Serena would have crumbled and Eeeve would have suffered an enormous setback.
As for battles? Whatever. It wasn't a deal-breaker for me. No matter her track record before the TF arc, Serena wasn't battling Lysandre. That was my problem with the Coordinators about how they were often portrayed on par with hardcore battlers. It didn't make sense. There is only so much time for practice. Of course, Contests were inherently flawed with the knockout clause. Heck, Ash would kick butt as a Coordinator. Five minutes to get a KO? No problem.
As an Amourshipper, I can't deny that the ship was mishandled. The writers treated it like click-bait too much. Ash and Serena missed out on more development because of it. Serena was hurt by its convoluted progress.
As for the 40 episodes without a goal. I was not really bothered by it. Serena still did things (unlike a red head and a breeder in Johto) in other episodes based around other characters. Besides, it felt realistic to have a main character without a goal. Misty had her water trainer thing, Dawn wanted to be a Coordinator, Iris wanted to be a Dragon Master and even May had a starting goal of traveling the world. The problem in hindsight is that XY and XY&Z is one of the shortest sagas, if it was AG or DP length, the gap wouldn't be so glaring. Plus, Korrina's quest stole episodes.
Overall, while I am more like Clemont in his geek ways, I feel empathy with Serena as I often give up, am defeatist and can be lazy. She is inspiring to me.
Was she or her arcs always written perfectly? No, but that is on the writers.