I imagine if you're born unbalanced, you learn how to balance it. Like at some point in the very early years of your life you crawled
so saying "There is just 60ish lbs more on the front it will fall over" makes zero sense. Maybe if it was a machine but even then the engineer can simply program "How about you lower the center of gravity at the back and place the front legs out a bit more"
Until trail and error it will balance (much like a child learning to walk and balance)
There are also people that do insane tricks with bikes, you just learn how to balance.
The problem is that the centaur has to move and move fast or there really is no point in being a centaur at all.
If you never move then you can get by fine, the problem is that moving requires you put the whole weight of your body on the front legs, lift your back legs off the ground, put them back onto the ground in front of where you were, lift your front legs off the ground and put them down again. If a creature is front-heavy we end up with a situation where it finds it cannot place it's back legs back on the ground and hence it ends up bowling over. It cannot learn to do anything except try to shuffle forward using only it's front legs, dragging it's useless back legs along with it.
The solution to the centaur problem is to ensure that nothing aside from the brain and mouth, the muscles needed to operate the above and some bones are found in the horse part. Have a very long oesophagus and wind pipe that goes into a stomach/lungs that are in the center of the horse part; the reason is that we want to increase the weight of the centre considerably compared to either the back or front parts. Place the creatures heart on the back of the horse part, just below the human part so that we can pump blood to the centaur's brain, this does result in a major weakspot which everyone who does not like centaurs will exploit; the blood must be pumped through massive arteries into the human part or else that large human-sized brain will not function, hence if anyone cuts the area at the back where the human and horse part joins we have a dead centaur.
The reason smaller creatures like insects can have multiple hearts is because they don't have a very efficient circulatory system to begin with. They are basically just sacks full of blood and the heart is basically a pulsating muscle that "stirs" the blood so that oxygen gets around the body. For an insect, having more hearts to stir the blood could be helpful. That's very different from a vertebrate circulatory system, where the heart forces the blood at high speeds through closed vessels. For a vertebrate to have multiple hearts they would have to be in sync with each other to keep the blood moving quickly. There isn't really much point when a stronger single heart would work just as well and give the body fewer points of failure.
I think if a centaur was a natural creature (and not a magically fused human and horse) there wouldn't be any reason for it to have redundant organs - giraffes have a head that is far higher above their heart and stomach than a centaur's would be (assuming the heart was in the lower body), but they don't need redundant organs (although they do have an unusually large heart), so why would a centaur need them?
Alternatively, the heart could be in the upper torso, making it easier to pump blood to the head. The torso could be filled up with a larger heart and lungs that extend to where a human's stomach would be, and the stomach/intestines could be in the "horse" part.
Also worth mentioning: Horses actually have an extra circulatory "assist" in their feet, called a "frog" - as the horse runs it squeezes the blood out of its legs and helps it to move around. (We sort of have a similar thing, actually, which is why it is more uncomfortable to stand in place for long periods of time than it is to walk). This has nothing to do with the size though; it is to help horses and humans run for long distances.
I would also guess from its body plan that it was either a predator or a browser, possibly both, but not a grazer. It would be hard for a centaur to bend down far enough to eat grass, but hands are nice for picking fruits or killing things. Hooves are good for running and since they can use weapons they don't need claws.
Keeping multiple hearts in sync with each-other is easy because vertebrate hearts are directly controlled by the brain.
The fact that creature's that have been around for millions of years fail to evolve organs that were not present in their ancestors even though they would greatly benefit from doing so and are instead forced to stretch their original body plan to breaking point is one of the reasons why Darwinism is rubbish. The original simple creatures all those millions of years ago have to have developed radically new body parts in order to end up with all the complexity that we see, but for some strange reason this ability stops manifesting at all in later creatures so all we see is the endless recycling of what we have been working with for hundreds of millions of years in many cases. All those hundreds of millions of years and endless recycling is all we see, but a long time ago and what we see is the emergence of a huge number of new systems in a few million years.
In any case if I was to go about making a centaur, a giraffe is actually a good starting point. We can make the humanoid torso out of the giraffe's elongated neck and make two more limbs out of the collar bones, supposedly limbs were originally evolved from ribs anyway. This does lead to the add situation where the humanoid torso has no ribs but instead neck vertebrae, but we can simply repurpose the size and shape of those so the function works out well, especially since there are no internal organs that need protecting.