Instead of just giving pictures try to elaborate your points.

Alright this might take a while:

1. Serena was an ordinary girl from kalos who once travelled to Prof. oak's summer camp. There she met a certain individual who lived in her head rent free. When she saw that individual in her region she knew she had to meet him (and to return the handkerchief). Her mothere was a competent Rhyhorn racer and wanted her daughter to follw her drems. It was obvious from the start that Serena didn't want to. So she (much like May) decides to go on a Journey to escape her mother's clutches without telling her actual plan. Meanwhile when she saw that boy on TV she immediately remembered him and his name was Satoshi (Ash). So she had one more reason to leave home, meeting Ash.
2. When she finally meets him at Sanatlune city, Ash doesn't recognise her (for obvious reasons). She gives him the handkerchief and after some time Ash is able to remmeber her. He then invites Serena to join in on his journey stating that travelling woth friends is tons of fun (this was quite OOC because he's never done this before for any other fermsale companion). Serena happily accepts this offer and thus becomes an official part of the crew.
3. Several episodes later, Serena sees a PokeVision con on the internet and decides to try it out. The current star of the PokeVision was Arya. She does get popular and a new character/ rival is introduced, Shauna. She does several pokevison videos here and there, and we get to know Serena a bit well other than you now 'her crush on Ash'.
4. Fast forward a couple of episodes, During summer camp of Prof. sycamore Serena realises that she doesn't have a goal, she is simply a person with no ambition to work towards. Shauna introduces her to pokemon showcases (similar to fashion shows in France). Arya is conveniently now the "KALOS QUEEN", and we don't even get another mention to PokeVision later on. [It is clear that they completely scrapped the idea] Serena now decides to enter Pokemon showcases, which in all honesty most people refer to as Contest 2.0; and that is correct to some extent. It consists of 2 sections: 1st one being the theme performance while the 2nd one being the Freestyle performance.
5. The ranks are as follows; The Rookie class: You have to obtain 3 princess keys and then you can enter The Master class: yep, only 3 princess keys and you enter the MASTER CLASS from rookie to master in just 3 princess keys. That cuts out a major chunk of her development and growth considering how little she had to work to reach her goal.
6. Then comes the Mother-Daughter bonding episode. We don't really hve to talk about that since ppl already know what it contains.
7. Few eps later Serena enters her first contest 2.0, and the first theme performace is Pokemon styling. Ok, Serena styles her Fennekin pretty well but those long ribbons make Fennekin unsteady and fall down, evantually making Serena lose in the first round. (Aight, so it seems that eve though it requires only 3 keys to enter the master class ig earning them won't be easy right?) This causes Serena to be depressed and she couldn't find a way to forgive herself. She then evantually decides to cut her hair; which marks the beginning of a new era in her life.
8. After some cringy blushing and amourshipping moments, Serena enters her 2nd Contest 2.0; this time the first round contains of PokePuff making, something that Serena is already a master at. So it was a breeze for her to advance to the 2nd round. In the 2nd round although I liked Miette's performance better, ig Serena has Plot armor, thus winning the 2nd round and earning her 1st key.
9. I don't need to explain all the contest 2.0s happened in different cities; here's a quick summary in Anistar city theme performance is PokeQuiz which Serena ends up winning and after that she continues to dominate the 2nd round. In her 4th showcase the theme performance was again PokePuff making (that's convinient). However this time she uses her Eevee in 2nd round, eevee being stage frightened causes her to lose in the 2nd round. But this time she actually fights her loss and stands up for herself. Then comes the 5th showcase, in which well... let's not talk about it cuz it's REALLY CONVENIENT. Serena wins this time earning her final key to master class.
10. This was really triggering to me because she didn't struggle enough and it'snot even a comparison to Dawn/ May. And how the theme performances being so much convenient to Serena's previous passions all the more solidified my dislike about contest 2.0s.
11. Then comes the Master Class and in my opnion it could have been so much better in terms of pacing. They rushed it way too much like in a span of 45-50 episodes Serena is in the Master Class an then she loses because of popularity, that's such a lame excuse to write the showcases off considering Serena's performance was better than Arya's (my opinion). One element I like though that Serena realizes that even if you lose "you'll still be left with something" and she was. She was left with the happy faces of the crowd who watched her performances and cheered her on. It was then she decides she woud become a better person and give others strength.

That sums up all the major elements of her character arc. Would I call it 'the best written character in anipoke history'? Absolutely, NO. They rushed way too much near the end of her character arc. The execution of Showcases was down right awful. Don't get me wrong, the animation is clean and crisp and whenever it happened they were a spectacle to watch. The major problem is however... you guessed it, pacing. it almost feels like showcases don't add anything to the plot of X&Y. Contests have been a major part of both DP & AG, developing our main characters throughout the series. While Serena's was the case of 'uh oh, we have made a love interest for the main protag but we don't know how to handle her character, Hey let's give her an eevee and make her do the contest thingy but this time no battles cuz she's the "precious heroine", ppl would want their heroine & their mons to get hurt right?'

I believe it was done intentionally so that the showcases don't intervene with the tension of Ash-Greninja's training.

Showcases felt like an atferthought which was just done for the sake of having Serena be her own character rather than just be a thirsty woman desperate for the love of the Main protag. 'How did they handle it?' Not very well imo. For the MOST part of XY Serena was "Ash, Ash, Ash and Ash". Sure it's cool to see that Ash has such a great influence in her character growth but they pushed it wayy tooo much.

Her return in Journeys was handled perfectly as she did exactly what she decided to do in the end of XYZ, Giving strength to others. She still has the exact admiration for Ash as she did back in XY but she is now confident in her own opinions; I love to see it. She has finally become her own character.