What you have said here is largely correct; in fact, it even contains most of the information necessary to refute your conclusion that co2 is ineffective as a greenhouse gas on Earth.
You are correct in saying that much of the Venusian warming (i.e., the warming above and beyond what would be expected only from its relative nearness to the sun), is due to the massiveness of its atmosphere. But, that massive atmosphere would mean nothing to the globally averaged surface temperature, if not for the presence of greenhouse gases such as co2 (and so2 and co and h2o and several others). In fact Venus has so much co2 in its atmosphere that most of the co2 spectral absorption lines are saturated. That means that even if we could add more co2 to the atmosphere of Venus, the surface temperature would actually change very little. This occurs partly because co2 is such an effective infrared absorber.
Now, on Earth, because the concentration of co2 is so low, the co2 lines are nowhere near saturated, for the most part. Thus, in contrast with Venus, adding more co2 to Earth's atmosphere does result in a significantly greater absorption of the upwelling infrared. On Mars, since the atmospheric pressure and temperature are so low, the co2 absorption lines are less broadened than they are even on Earth, despite -- as has been hinted at elsewhere above -- the fact that Mars has more co2 than the Earth. Thus, Mars experiences very little greenhouse warming.
The point of all of this is that you are right to say that simply looking at the amount of co2 (or the amount of greenhouse gases in general) in an atmosphere does not tell you whether the planet's surface should be warmed significantly by a greenhouse effect. However, the reasons for why that is insufficient is precisely the reason why the increase of co2 in Earth's atmosphere, and its potential impact on the climate, must be understood.
And finally, to clarify some of your general statements above, the surface pressure of Venus is just over 90 times Earth's. Greenhouse gases (which include h2o) make up about 5% of Earth's atmosphere. And the temperature at the surface of Venus, if you assume that it has the same atmospheric make up as the Earth, so that the only difference is the increase in the amount of solar energy received due to its nearness to the Sun, would be only about 60C, compared with the 450C of the "real" Venus.