Marx himself believed that socialism was only possible after a capitalist phase and that it was precisely capitalism’s tendencies that would make socialism appear, and in turn give way to communism eventually.

(Emphasis mine.)

This isn't quite true. Marx believed revolution would happen in developed capitalist economies first, but not that this was absolute. See his famous letter to Otechestvenniye Zapiski:

Be that as it may, as I do not like to leave anything to “guesswork”, I shall speak straight out. In order to reach an informed judgment of the economic development of contemporary Russia, I learned Russian and then spent several long years studying official publications and others with a bearing on this subject. I have arrived at this result: if Russia continues along the path it has followed since 1861, it will miss the finest chance that history has ever offered to a nation, only to undergo all the fatal vicissitudes of the capitalist system.

Marx did cross out this paragraph, but you can see that Marx did not hold such a rigid, formulaic view of the progression in modes of production. That said, whatever came to Russia even without going through capitalism would still have needed to go through socialism to reach communism, they just did not need a period of bourgeois domination and profit above all.