How fast can a Drama go downhill?

This is my first review, so bear with me.
I enjoyed the first episodes, the characters were mostly complex, they had a goal and the leads had chemistry.

I would say everything went downhill when he colored his hair brown (Why is that such a common trope? Could the leads please keep their colored hair, they always look so good. Nvm). Even in the first ep. he had abusive tendencies and there I already knew that my rating wouldnt go above 9, but sometimes they have such a good chemistry that I will watch it even though I mostly despise these scenes. And ohh man they had chemistry, The first 4 to 5 episodes they had so much skin ship and banter that i was ready and happy for more in the later episodes, but suddenly it stopped and with it their chemistry.
Suddenly all they did was scream at each other and be jealous little kids. Especially his screams and anger tantrums were extreme and I hated them so much.
The lead actors and the sfl were all very good actors and they potrayed emotions very well but in the later half the script didnt do them justice. The rest were mediocre to even bad, especially Kangnam (As if there are not enough thai actors or even idols that could act better, but understandably if no one wanted to play this stereotypical character.) did an incredibly bad job.
The rl were so intriguing in the first ep. and the sfl had an enough likable character for the typical 2010s sfl, but suddenly everyone changed for the worst, no one had goals anymore. Why doesnt he want to finland anymore, why doesnt she want to cook anymore?
And were went the skin ship? Like the accidentally touched in the first 4 episodes more than in the rest of the episodes combined. Also there were so many characters and scenes that were totally unrelated to character or plot development.
I will add more when I thought about it more, but in conclusion: This is a disappointment that went downhill the farther we got into the series.

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