Seeing this makes me miss the salad days of MOOCs. I learned programming in the 2010s through MIT’s EDX Introduction to Programming course, and then a course on Coursera by Martin Odersky on Functional Programming through EPFL, and I feel like that ladder has been kicked away due to MOOC monetization policies. I wonder if we could return to these days.
* memory safety and exploitation (the "buffer overflow" section is about 20 years out of date, though super appropriate for a first course)
* the WebPKI/certificates thing
* messaging security and messaging cryptosystems,
* microarchitectural security and hardware side channels.
Multiple full courses on each of these subjects would bring you up to "practitioner" levels of expertise.