Thanks, and this may surprise you, but the fact that you like the art and world building is very encouraging. Without spoilers, the plot really starts in the next chapter. So if you find the world interesting, that was the goal of the first three chapters. Hopefully the plot is compelling as well.
That's the problem. The story should start not with the next chapter, but with the very beginning of the game.
What are the goals of the first chapters? I suppose they are to show the world in which the protagonist lives and his motivation. Namely, why does the protagonist so desperately want to discover his element?
The protagonist lives in a world of magic, where everyone possesses magic to some extent, but this is not shown in any way. Therefore, it is completely unclear why we are constantly forced to traipse around libraries in search of information.
How can this be fixed?
It's actually pretty simple, and you don't even need to change the plot. You just need to add some illustrations in the form of comics.
For example:
1. After defeating the slime queen. The older brother's sword is covered with ice, then he strikes the slime and it freezes. Then the second brother's weapon begins to sparkle with lightning bolts and he smashes the frozen monster.
2. After defeat at the dance. One of the girls snaps her fingers and a stream of water pours over the insolent guy. After that, they tell him to cool down.
3. After defeating the bandits. One of the girls casts a spell on the bandits, who get scared and run away.
4. After defeating a guy while duck hunting. The partner's shield begins to sparkle, the guy hits it with a spear, and his weapon breaks.
5. During breaks. Show any use of magic in everyday life. Filling a cauldron with water pouring from the air, or starting a fire with a snap of the fingers. Or maybe setting up a tent with the help of wind magic.
It is also worth adding special powers to the companions. For example, like the healer girl. In addition to her, we will team up with three other people. So why not give the girl from the first city the ability to increase her defense. Not much, just give her a few extra defense points. At the same time, the brothers should receive abilities that increase their attacks with one element or another. Let's say +10 damage with fire or electricity.
Even the first chapters should tell the story of the main character. His aspirations and experiences. Reveal the environment and problems. And this should be expressed not only in the plot, but also in the gameplay.