E=mc^2 is the expression relating the rest mass of an object to it's energy equivalent- objects can also have kinetic energy in addition to the rest mass energy.
For the TOTAL energy of an object the rigorously correct equation is E = γmc2
where γ=1/(sqrt(1-v^2/c^2)) when the object is at rest, v=0 so γ=1 and the equation becomes E=mc^2.
Einstein did not just pick letters and constants at random until he got something he thought looked right.
He started with the assumption that the laws of physics were valid for all reference frames and working from that the math and physics dictated that E=γmc^2. He had no choice in how it turned out, YOU have no choice in how the equation turned out- it simply is the way the universe IS.
You seem to think it needs to be changed- but the universe doesn't give a damn what you think.
If c^2= 300,000 km/s * 300,000 km/s = 9*10^10 km/s
nope- you still have your units wrong -velocity*velocity will have units of m^2/s^2.
Energy has units of Kg *m^2/s^2
That is why the units of Einstein's equation make sense, and the units of your nonsense are just gibberish.
Johannes, there is no shortcut to understanding the science- you can learn the math and physics and understand how it was developed by generations of great minds, and appreciate its beauty- but you can't be lazy and ignorant and just assert you have a better way and expect anyone to think you a sane individual.