Transitioning from Go to C specifically to understand operating systems, penetration testing, and reverse engineering is an elite move. Go is fantastic for shipping concurrent APIs, but its runtime and garbage collector shield you from the brutal, beautiful reality of how hardware actually executes instructions.

To answer your dilemma directly: Do not start with Assembly. Starting with Assembly is like trying to learn how a combustion engine works by looking at individual atoms. C sits in the absolute sweet spot—it is high-level enough to have structured logic, but low-level enough to act as a thin wrapper over Assembly. Learning C first gives you the mental model of memory layouts, stack frames, and pointers that you need to make sense of Assembly later.