Good bit, lol!
Lenin's advances on Marxism are largely in analysis of imperialism, and practical revolutionary tactics such as democratic centralism, as well as to combat the revisionism of the second international. Trotskyists often try to call themselves "Leninists" to differentiate themselves from Marxism-Leninism, or "Marxist and Leninist," but their own theory diverges from Marx's.
Lenin's contributions are not diversions from Marxism, but additions and updates to follow the development of imperialism, which pushed the contradictions Marx thought would lead to revolution in Europe into the global south, and all the new questions that added.
For anyone that wants an intro to Marxism-Leninism, I made an intro ML reading list you can check out!