Ask HN: How to increase depth instead of breadth as 10 yoe as swe?

A software engineer with 10 years of experience (C, C++, Python, GPU/DSP programming) feels they have broad knowledge but lack deep expertise, partly due to working on varied customer projects at the same automotive supplier for 8 years. They are struggling with job confidence and marketability, and are seeking advice on how to develop deeper specialization and overcome this career plateau.