TL;DR: A Democratic administration is marginally better for Iran in the short term, but it's the US relationship with China and the choice between US-dominated unilateralism and Chinese-led multilateralism that will ultimately shape the world's relationship to Iran in the long term. This is largely a structural issue with the bipartisan defense establishment which controls the military-industrial-congressional complex.

There are different factions in the US security state that are interested in doing imperialism differently. The Iran hawks are concentrated in the GOP and many of them are Christian Zionist maniacs who want to hasten the end of the world so Jesus can come back. A Democratic administration is more likely to engage constructively with Iran, but they're still imperialists, so that engagement will still be on their terms.

I think Biden's foreign policy depends on whether he gets Obama second term people who are cynical realists interested in maintaining balances of power (with the US as a more restrained but still hegemonic power), more hawkish on China than anywhere else, but also wary of appearing like an aggressor, or Obama first term/Clinton people, who are humanitarian interventionists (basically blue neocons) that believe the US can promote a moral mission of remaking the world in its own image. The Iran deal and the Cuba thaw are emblematic of second term Obama foreign policy and are like the only good things Obama did. The second term group is more likely to favor "low footprint" "safe" interventions that don't risk a lot of resources; the first term group is more likely to favor "boots on the ground." Obama's second term saw a continued increase in the drone program and sanctions regime. But I think those people are more likely to see sanctions as a bargaining chip for diplomacy and not (as much of) a means of starving people into regime change.

I think the "pivot to Asia" has been cemented permanently, it's just going to be an issue of how the US contests China in the region. I think the relationships to the smaller powers in the region like Vietnam and the Philippines will be more important (and Vietnam's turn to the US will frustrate Western leftists who want to act as if any socialist project has somehow abolished the nation state form). If the US tries a conventional cold war against China, it will eventually lose because China has a much larger capacity for defense R&D and production than the USSR, and will eventually surpass the US in this regard. But adopting the cold war mentality will benefit the careers of a lot of people in the defense/foreign policy establishment in the short term. It will be an impetus for increased defense spending on both materiel and research, which benefits all parts of the military-industrial-congressional-academic complex. There are smarter approaches to attempting to contain China (like exporting A2/AD technology to China's neighbors), but they are not as lucrative for the defense establishment so it appears the establishment line, at least for the time being, will be cold war arms race and a (failed) attempt at economic decoupling.

But the US's allies from the first Cold War will not follow it into a Manichean struggle against China. China does not pose an ideological threat to their ruling classes like the USSR did, and will always be constrained in its ability to project power beyond their backyard. The US will only get full sign-on from the other Five Eyes who have a cultural affinity for each other. The US establishment will be unpleasantly surprised by the limits of the willingness of their other traditional allies to "oppose China". The US's insistence on a "with us or against us" attitude will undermine its influence on the rest of the world, which will create more openings for the US to be ignored in other areas. Countries which once would have gone along with sanctions against Iran will refuse to do so, because they won't be getting much from the US in return. The fight between a US-controlled unilateralism and a China-led multilateralism will ultimately settle in favor of multilateralism, because it gives smaller powers more freedom. This will eventually result in the degradation of the US's ability to punish countries like Iran.