I'm yet to read any convincing account about how, if we got there, Leninists would start to break down the vehicle they used to defeat capitalism and rallied society around.

Lenin himself writes extensively on critiquing the existence of the state, so any real leninist would align with this position. Mao himself also was quite anti-state, see the cultural revolution. Both of these incredibly important figures were anti state but saw it as a necessary tool to defend against extreme capitalist agression. there is little doubt in my mind that if capitalism fell, many would start critquing and deconstructing the current power structures.

Many would see it in their best interest too, because every head of Marxism leninism has shouted to the stars about the bureaucratic state apparatus corrupting Socialism and causing capitalist restoration.

Doesn't it seem reasonable that the idea of trying to create a classless society would be a task better suited for those who come after us? For example, the promise of socialism is that democracy is then increased to be available to the wider masses of people. Socialist societies have in the past, and even in the modern day, demonstrate that they're able to make huge leaps in social progress that enable more people than ever before be able to participate in the democratic processes of a society. I guess I'm not convinced that the idea that "Leninsts would not break down their own state" is something which is provable, and thus not a useful heuristic for making decisions. So what if "Leninists" aren't capable of the next step in the growth of humanity, it's been shown that they'll give up on their power much more peacefully than societies dominated by the bourgeois class ever will (even and especially communist officials who didn't benefit from the transition to liberal governance). If we're able to save the planetary ecosystem with cybernetic planning, end hunger, guarantee housing and work for those who're able and a good life for those who aren't as the "Leninists" demand; won't we have left our children with far more fertile soil for an anarchist society than if we simply struggle directly for a classeless society today?

whose primary functions include reproducing itself

Mind expanding on that since it seems like in the us its doing a piss poor job of it. In fact I'd say that a capital dominated state wants to do away with itself via privatization.

Engle's makes a pretty convincing argument that the state arose to mediate class conflict in favor of capital and that it cannot be destroyed until that class conflict is resolved.

Take the landlord tenant relationship. In order for this to exist then the landlord must exercise their property right through state mediated violence and the tenant is offered some rudimentary protections. If the state simply no longer recognized the property right of the land lord the state would wither and class conflict would resolve a bit.