Do you think a "better'" planned planet can be assembled...?

Not for us...
Each lifeform is going to evolve and adapt until it is perfectly adapted to its environment. For us, that is this planet.
If our planet had more gravity we would probably be stronger.
If our planet were colder we would have thicker flesh and hair.

Now think even further. What if we didn't evolve on a planet at all, but in a nebula?

At any rate, there is no "better" planet, because no matter what, it would still be an opinion; Our opinion. I am sure that the locals of that planet would tell us that their planet was better. And no matter what, after a few million years, any planet that a species has evolved on will be perfect for them, as long as the nature of the planet doesn't change too often.

I think I read somewhere that if all the asteroids in the belt were reassembled, they would still make a body smaller than the moon.

One would have to consider the proximity of said planet to the sun. Getting the correct temperature would be vital to sustaining life, and there would be no telling what size the planet would actually be. Its added gravity may adversely affect other celestial bodies.

Or something like that.

Is this a serious question?