If Iran got close to creating a Nuke wouldn't the US just bomb the site? Iran must surely know that it will not get away with building nukes either. So what are they doing?

Iran's bark is larger then it's bite. I think it just wants to use its nuclear activities as a bargaining chip for sanction relief.

So why should the US do anything at all regarding Iran? It can destroy those sites easily enough if it needs to.

Why doesn't it just refuse to negotiate and keep Iran poor?

Iran's military threat are, relatively, a mild annoyance, and calling their bluff and immediately escalating as trump did when he killed Soleimani seems to temper them as any escalation hurts them a lot more then it does the US.

I think when we treat Iran as a real threat, as we did with NK, it just gives them more legitimacy then they deserve.