Having realised the destructive nature of humans, especially in the post atomic and nuclear age, this Mycelium intelligence would see the need to neutralise the human threat. It would be acting in the interest of self-preservation, not with malicious intent. And as such, I don't think it would naturally resort to violence as the means to this end.
It is possible that they would survive a nuclear apocalypse, as we've seen post Chernobyl and Fukushima, but that option would cause too much damage to the ecosystem which it is inherently connected to. Hence, any path that leads to this would not be the natural choice of such a connected and enlightened entity.
Rather than aggravate humans to self destruct, and risk demolishing a large part if not all of Earth with it, a better option is to placate humanity.
Psilocybin is already found in more than 200 species of fungi. If it can leverage ways to make its psychoactive compounds more readily accessible to human minds, by direct inhalation of the released spores without needing to ingest or steep the shrooms in a tea and such, that would already be highly effective.
As a conscious intelligence, it can start directing its resources to propagate these Psilocybe species as widely as possible. From there, the possibilities are endless.
The most aggressive option is to spread the most psychoactive species of mushrooms as widely as possible, so that humans get high on LSD and DMT trips. Addicted even, that they crave the mind altering effects and lingering euphoria, so that mask mandates by those still sound of mind to rein in these global psilocybin epidemic, fails.
Humans most exposed to its effects lose their sense of time, and stay hours and days in an active trip while out in the wild. Forget jobs. Forget life. Forget eating. You're not hungry anyway. You feel alive. Connected. To the trees around you. The earth beneath you. To the soil above you. You have returned to where you belong, to Mother Earth. You are one with the Mycelium. You feel the need to reach out your tendrils to share your spiritual experience with others of your species. Let them know you're waiting for them to join you in this plane of existence of eternal euphoria.
All it needs is to infect a significant percentage of humanity at the onset. Some will feel the effects more extremely than others, of course. Not all of humanity needs to serve directly as the primal food source of the Mycelium. That is just a welcome side effect to fuel the Mycelium's longer term plan to reshape the world.
As the hippie culture kicks off, and with already a large part of the population lost to the more extreme effects of psilocybin, society will start to collapse as integral systems of our civilisation that is dependent on human cogs fail one by one. Supply chains falter. Manufacturing slows. Food supply dwindles. Communities isolate. Communication and power systems age and fail over decades and centuries of neglect as the global machinery of civilisation struggles to adjust to the aftermath of population collapse.
During that time, more and more people will see the appeal of surrendering to the calls of the higher beings they meet while under the influence of psilocybin. Why toil in misery and struggle with sorrow when there is eternal bliss waiting to welcome you to its fold?
Over time, the jungle will reclaim concrete. Sands of time will erode the steel skyscrapers that once towered over trees. The surviving tribes of people will tell stories around campfires of an age long gone when insolent humans deigned to rule the Earth and did not honour the Gods as they should.
Another less nefarious option is simply by making edible mushroom species carry more and more trace amounts of tryptamine alkaloids. The shift is gradual at first, making mushrooms a mood enhancing superfood. No need to raise the alarm. We see less conflict and violence. Less debilitating depression is a good thing. A new normal embraced by all. But over time, a euphoric population slows innovation. When people are content, there's less hunger, less drive, less need to consume the world and its resources to fill the empty void within. Development slows. Deforestation is halted in its tracks as humans reconnect with each other, and with Mother Earth through LSD/DMT trips. Threat neutralised.
It's effective, but the writing needs to work harder to make the narrative compelling if that's the extent of the Mycelium's plans. But this could potentially occur with the support of a cordyceps-like mushroom with the neural epigenetic remodelling capabilities of taxoplasmosis gondii. The Mycellium can begin to alter the behaviour of humans to be more docile, earth-friendly, and less keen to procreate. It doesn't need to wipe out humanity per se, just retool the species for other purposes. Maybe even actively operate their human host species to then colonise other worlds and infect other species.
I like your self-aware Mycelium premise. The potential seems, limitless. Good luck with your story!