
The main character's favorite thing in the world besides her family is singing, she was in a horrible world where the only out was singing, the songs she sings are at risk of being illegal and her damn power is singing.
It's a story about singing and the wit of the heroine, not a cheap quick love story that will make your heart jump because of overplayed cliches.
Don't get me wrong, I love those stories! I read them over and over again and live watching the main characters fall in love at first sight, but for the love of god, could you people not compare those quick fluffy stories to stories with actual plots?
There are stakes here, the heroine main struggle isn't "this is an arrenged marriage, but i like him?!(´ε` ) ", she watched her country go into ruin and all of her family die. Don't read Lady Baby, The King and his knight or Yeon Lok Hein if you want a cheesy love story. The slow pace isn't a bad thing, the focus on the heroine's journey instead of her romantic feelings isn't a bad thing and the fact that it'll take a long time for the proper romance to come isn't a bad thing!

you're not wrong. singing is something at stake. but at 13 years, the priority should not be changing societal standards of vulgarity. if this were more realistic, her opinions wouldnt be considered with her age and sex. it's not realistic to prioritize religious viewpoints over murder. it isnt logical. she's just bringing more attention to her family, making more reasons to be a target....

Are we reading the same story??
She is changing the standards of vulgarity, because the shame used by the temple was a way to control people, that's why all those people died.
How could the folk songs be illegal for any valid reason? They're expression of one's self. The reason why they were made illegal is because singing about anything else besides the hymns that praise the temple and what it stands for could undermine the temple's power.
No real antagonist is also untrue, we get minor antagonists sprinkled in because the biggest problem isn't one person, it's the controlling world the heroine lives in.
And I'm not sure whether I should even justify "fruitless protagonist" with an answer. Just because she isn't bubbly doesn't mean she's boring. She holds herself to a high standard and doesn't let anyone trample over her. Normally in these stories the male lead would come out of nowhere to save the mc, but she stands her ground. She's witty and cunning.

your points aren't wrong, but, in my perspective, music and singing should not be her main priority right now if she aims to immerse different culture and undermine the temple. her methods are unrealistic. i call her fruitless because her acrions and responses are predictable at this point, not because she's not "bubbly." her character does not live up to the potential she displayed in earlier chapters. it's fruitless because there just doesnt seem to be any more growth for her anymore. she's always gonna be that witty kid that one-ups everyone around her and nothing can stop her because this is her story.
the antagonist part... yes there's minor ones, but they were used as a ploy to emphasize the lead's intelligence and shit or wring up more drama.
the story lost its logic

Usually in these kinds of settings a perfect man is someone attractive, cold and good at his job, but he's an asshole. Section chief is actually what I'd picture under that term. He isn't cold, he's just professional and when in non professional setting, he's a dear! Like have you seen all those blushes and little smiles??! So loveable! He asks for consent, but isn't afraid to be spicy. And it's not like he's a flawless blank slate, he didn't understand what mc was going through, lashed out because it seemed irrational to him and then he doesn't apologize because of this, bit promises to try and understand mc.
And the mc! Oh gosh the mc!
He's also amazing, i really don't like it when the main character is incompetent, so i really like him. I also like that he's 27, his brain is fully developed so i don't gag that he's dating his superior. He takes the harsh treatment in the professional setting, but when section chief insulted him, he didn't back down and didn't allow him to get treated in such a way.

I really like how the story isn't playing too hard into gender norms. The master is kind of feminine, but everyone still respects him and there have been introduced many strong women and only one of them could be a love interest (or maybe she'll be just an ally! That'd be really surprising, but not weird.)
Sometimes when designing characters, the artist tends to give guys the interesting design and the women the pretty design.

HER NAME IS LITERALLY LESBIAN WITHOUT THE B?!!?!!
She'd literally be so cute if she went the maid route! I mean look at how they get along! They curb have personalities which is more than you can say for some ships in this genre.
She's the sword lesbian all the other lesbians day dream about!! She could even have her own harem here. Just imagine, they run away together and while Leonie gets a job as a maid somewhere else, Lesian becomes a knight!! And how fun would it be if one of marquis's daughter was divorced and the other was gay. Perfect revenge on him by fate.

The plot: The emperor abused and manipulated the mc since he was a child, now the wiser and more powerful mc came back in time to change his fate!
Me: Awesome! So he kills the absuive emperor and falls in love with an ally who treats him as his equal?
The fucking plot: ◉‿◉
Me: He, he kills the emperor right??

I usually feel the same as your comment on most of these kind of stories (Yaoi or shoujo) but I forgive this one because 1. the ml is not human and grew up in a messed up culture with zero affection or anyone explaining emotions and how to to deal with them or just basic human decency. He changes in the second life because this time he grows up with MC and he taughs him all this (unlike the shitty abandoned Empress where the ml has absolutely no excuse for being a piece of shit)
The recommend lists seem to be a bit contradictory on this one, but I'll give it a shot!