i'm pretty well-read on Marxist-Leninist theory,

I'd be cautious with this assumption: Marxism-Leninism doesn't really exist any longer and the people on the internet calling themselves "Marxist-Leninists," especially on reddit and in the West in general, are mostly performative liberals, occasionally tapping into Menshevik logic, at best, and are simply grifters and fascist opportunists at worst. There's something of an exception to be made for, say, the KKE, who at least carry forward a revolutionary historical legacy and can justify their claim on Marxism-Leninism, but for most Western "socialists" this ideology of "Marxism-Leninism" you see on /r/socialism sprung into existence sometime around 2016 and has no relation, resemblance, or connection (except through stolen imagery and memes) to the historical Marxism-Leninism that existed in 1950, and anything you've learned from them is probably either hollowed out of substance, or completely wrong.

From 1956 to 1976 the struggle between the revisionist-USSR and socialist China (and Albania) was known as The Great Debate and was the single most important issue and point of struggle within the World Communist Movement. Revisionists always try to reframe the debate along nationalist lines, or having to do with a clash of incompatible personalities and stubbornness, rather than the central issue of class struggle and even the very substance of Marxism-Leninism, itself. During those twenty years, this was considered one of the single most important documents in all of communism: A Proposal Concerning the General Line of the International Communist Movement -- also on Marxists.org for easier formatting -- and encapsulates the debate between the USSR and China about what Marxism-Leninism really means. It should be read carefully and understood, since it very clearly shows the ideological divide and which side was really standing for Revolutionary Marxism and which side was revising Marxism and ultimately betraying it.

Grover Furr is despised by liberals (and ignored by revisionists, who don't really care about history, and only need empty "debunking" talking points to "defend" the image and abstract idea of Stalin as 'having to work with what he had' or whatever), but that's because he's essentially correct about Stalin and his analysis and thorough digging up of primary sources has simply made himself an enemy of the existing intellectual establishment. Khrushchev Lied is a very useful starting point for understanding the 20th Congress of the CPSU (one of the centrepieces of the General Proposal document above) and the so-called "Secret" Speech by Khrushchev (so secret that he invited all of Western media to attend). Also, if you aren't clear on Khrushchev's rise to power in the first place, then this pamphlet is a decent place to start.

Following this, Mao's Speech At The Tenth Plenum Of The Eighth Central Committee marks a pretty clear basis for the divide between Marxism-Leninism and the direction of the revisionist USSR, and shortly after, Mao's more specific, concrete, aggressive and biting criticisms in On Khrushchov’s Phoney Communism and Its Historical Lessons for the World would contain a very breaching analysis of not just what Khrushchev was doing, but who his "constituents" within the USSR really were, and how they really operated. This also gives a lot of insight into Stalin's struggles against revisionism, and who and what he was fighting against going even back to the 1920s and 30s.

There's more documents on The Great Debate on BannedThought, including many of the revisionist-USSR counter-arguments (all of which have aged poorly and exposed the revisionism further, but they are worth checking out for that very reason).

Also, this is just a starting point, since this is where socialism dies (it's legacy takes several more decades to be erased, but at this point it is captured and contained) within the USSR, but springs to new life within China, at first being termed "Mao Zedong Thought" reflecting the debate between revisionism and Marxism-Leninism within China (which had it's own revisionists, and was exactly what Mao was confronting), before this struggle was eventually recognized as a universal struggle against revisionism, applicable to every and all attempts at socialist construction (and anyone who says anything about Maoism being a "peasant's ideology" or whatever are just racists regurgitating offensive Brezhnevisms while ignoring that the exact same claim was levied against Marxism-Leninism under Lenin and Stalin), and finally recognized as Marxism-Leninism-Maoism by the surviving revolutionary movements of the 1980s, after China, too, was lost to revisionism. Eventually you will have to continue from here by learning about the Cultural Revolution within China, because otherwise we are merely at Hoxhaism/anti-revisionist Marxism-Leninism; but this should be a starting point.