Ask Lemmygrad

But what exactly caused the split?

You sort of answered this one yourself. Among numerous factors it is precisely the things you mentioned - the de-Stalinisation nonsense, the purge against pro-Stalin elements (if you can even call them that - they were just anti-revisionist Marxists-Leninists), blatant revisionism of Marxism-Leninism and its core principles (continuous class struggle, dictatorship of the proletariat, the party of the proletariat), blind optimism when it comes to national question

What policies did he push that were reformist or capitalist in nature ? How exactly did he fuck up?

They weren't all strictly capitalist in nature, to be honest. It would be more accurate to say that his blunderous policies created conditions for capitalist restoration inside Soviet Socialism.

The policies typically referred are such: he dismantled the state-owned MTSs (machine and tractor stations), putting the responsibility of maintaining and repairing the machinery on collective farms. He dismantled central planning, replacing existing institutions with decentralized regional planning committees, which greatly exacerbated the existing difficulties with planning. He encouraged the peasantry to keep more privately-owned produce and livestock, essentially strengthening NEP-style measures without second thought. He adopted wage-leveling - a mistake of monstrous proportions, which decimated incentive for production growth (more of a left-deviation, honestly - the USSR was not ready for such a thing), and also created severe discontent among the intellectuals, prompting them to look for other means of enrichment, siphoning this strata of society into the "second" economy who would then constitute would-be capitalists in its embryonic form.

He also started the Virgin Lands cultivation bullshit, instead of trying to make a qualitative shift in agriculture. The idea was also to emulate US agricultural practice with heavy use of mineral fertilizer. The results were disastrous, partly due to the fact that initial yield seemed to have increased (but that was only true for land already cultivated), which gave overall sense of false promise, and also due to abandonment of Stalin's afforestation program, which worsened issues with droughts.

There were also big mistakes of political nature on top of those related to the economy, including the damage done to CPSU - recruiting too many people of questionable ideological strength, massively increasing the % of intelligentsia compared to industrial proletariat, needless bureaucratization, etc. He also drove a split into industrial and agricultural factions inside the CPSU.

Simply said, his overall strategy represented a Bukharinist right-deviation within the political spectrum of the CPSU. Something Stalin warned might happen in a peasant-dominated country.

The list is hella incomplete, please feel free to add more stuff.