This is indeed a passage that is easily misinterpreted by many of its readers. A key to interpreting it correctly, is to note whom Christ is addressing when he uses the expression in question. _It isn't the rich man._ The rich man had already left, because he was not prepared to prioritize God over material riches. The majority of people listening were not rich. They were the common folk. The lesson is, as you've made clear IMO, that the greatest good is God, and prioritizing other goods above God is to make a god of lesser goods, and that this does not lead to the ultimate happiness that is what Heaven is by definition, which consists ultimately of unity with the authentic greatest good first and foremost. And indeed, as other verses state, those who prioritize lesser goods will ultimately have neither the greatest good nor lesser goods. Everyday experience tends to agree with this as well. Obsessive pursuit of lesser goods (which are indeed good) often makes those goods inaccessible to us. We lose on all fronts.

And so, in a very real sense, as you also note, there is nothing especially important about the rich man per se. The poor can lust just as much, if not more, for money than the rich, because the poor can more easily fantasize about and project hopes onto what they do not have; the rich man is more likely to have become disillusioned with his riches.