I wouldn't say Marx is outdated, his analysis and frameworks still hold up. He did not live to see the extent to which Capitalism would shift towards Imperialism, but Lenin and other Marxists consistently applied the frameworks laid out by Marx. I guess this is what you mean when you say you'd be missing foundational pieces by missing Marx, however, so I may be splitting hairs.

In my opinion, Marx is crucial for understanding the basics of political economy, Engels does a great job of simplifying dialectical materialism, Lenin forms the basis for understanding Imperialism, and Mao is the best source for practical applications of theory. Stalin's works are very easy to read and can be handy, but focusing on Stalin's works over Marx, Engels, Lenin, and even Mao feels... off. I would recommend reading all of the texts here eventually, but it doesn't make a concerted effort to teach the real aspects of Marxism in a practical manner, it reads more like a "best of" list than an actual training and learning regimen.

Just my 2 cents as someone with their own reading list.