A decade on systems that couldn't fail didn't teach you syntax it taught you consequences. That's rarer than any skill on your resume. You need harder problems and primary sources not more material. Kleppmann's Designing Data-Intensive Applications if you're going data. Ousterhout's A Philosophy of Software Design if you're going engineering. Then pick one system you respect, read its internals, and form opinions. The next level of growth doesn't come with a syllabus. You have to construct it yourself. That's not a gap. that's what senior actually means.