For dev roles, Python experts (emphasis on expert) and experts in modern C++ have the easiest time. C experts and systems programmers with assembly expertise can also find a job, but the market is smaller for them (although higher-paying).
OS-level and systems programming are valuable. Understanding computer architecture is important, and computer systems as well.
Credentials are an important filtering mechanism for them, so if you didn't go to a top 10 school, your resume needs to make up for it (or you need to be a very experienced hire). Working at a FAANG company or a bank doesn't necessarily make up for not going to a top school.
Successful devs at HFTs usually have a ~50-hour workweek or are very efficient programmers.