> they won't do it because they're rational actors who understand mutually assured destruction

It's both. The Iranians are rational within their own cultural understanding of what rational actions are. It's not rational, for example, for them to build nuclear weapons, deprive their people of economic prosperity, kill thousands of protestors, fund militant groups spreading terrorism and murder throughout the region, yet they do it anyway.

You can be simultaneously "rational" in some ways and "irrational" in others. Their general activities are irrational, but that doesn't mean they see the point in launching indiscriminate missiles at Israel only to be on the receiving end of a nuclear weapon. Why not wait until you have dozens and then go for it and launch one at Europe (don't forget, despite European politicians kissing the toes of Iran out of fear and cultural decline that Iran still hates Europe too) or the US too?

> could there be other reasons for Iran to pursue nuclear weapons (i.e. sovereignty and national security)?

Why do you assume the other reasons are for peaceful purposes? Can you think of some malicious purposes that Iran might acquire a nuclear weapon for?