Mars already has an atmosphere as thick as earth's, its just colder and too dry.
Not even close. From Wikipedia: "The atmospheric pressure on the Martian surface averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure of 101.3 kilopascals (14.69 psi)".
Terraforming Mars would probably never fully work because it wouldn't be able to hold onto its atmosphere like Earth can. The solar winds are constantly blowing it away and making it thinner, because Mars has no magnetosphere, which protects our atmosphere from the same fate. To get it to be sustainably habitable again you would need to restart its core to generate a magnetic field, which would probably take a massive amount of nuclear weapons or something on that magnitute. Like more nuclear weapons than have ever been produced by humans up to this point.
