I also love reading Engels, and Anti-Duhring is every bit as important of a theoretical basis as a work like Capital. Capital achieves in practical analysis what Anti-Duhring achieves in theoretical analysis.

Economic and Philosophic Manuscripts of 1844 I think is Marx at his most digestible although he's much hazier on the mechanics of capitalist appropriation than in later works. Thesis on Feuerbach is a work of Marx's that is extremely concise and IMO understudied and poorly understood if not outright ignored. In a few hundred words Marx thoroughly obliterates any notion that he never considered "human nature" in his analysis, on the contrary his entire analysis begins with and stays permanently grounded in his deep love and optimism and hope for the spirit of humanity and our development; our immense potential for self actualization through revolutionary socialist cooperation.

There's just so much that can be learned from Marx specifically, and even if his later work is complicated, it can be understood best when you work with a group to understand it. It isn't something for the individual to master, it is for us all to free ourselves. But Engels in his own right, as difficult as it is to separate them, is a fantastic writer and theoretician.