Why does the Dwarf in the Flask want to be a perfect being, when he's already an immortal, and has the power to do what he wants using the Seven Deadly Sins (Lust, Envy, etc.) homunculi?
... had the power to do what he want...
Actually, this is not true. You're suggesting that he has the things he wants already (control of the country, great alchemical power, and so on). But the reality is, Father wants much more than this.
Father's true motive is, in fact, to be free from the laws of the universe. Because of his upbringing being sealed in the flask, he grew a need to constantly free himself from things he perceived as bounds.
This motive is stated on the Fullmetal Alchemist Wiki:
When confronted by Hohenheim about this, Father reveals that the seven Homunculi were only created so he would become a perfect being. Ultimately, in his final moments, Father's motivation is revealed to be freedom from the laws of the universe itself.
And is backed by various quotes from the series, including the following:
[I wanted to live] in this vast world without anything being able to hold [me] back!
— Father, Brotherhood episode 63, 6:20
In his mind, not only were the people of Amestris (and the world) holding him back, but the physical laws of the world were binding him. Just as he formed his inferiority complex while restricted to the flask, he also felt that the world was restricting him from obtaining the freedom he wanted.
In order for him to feel this freedom, he had to discard and surpass the laws of the universe; in turn, he had to absorb and control that which created him: 'God'. This is why he wanted to become the "perfect being".