There are probably a few effective approaches. One is to get another degree. That looks like a career reset. The easiest is probably Open Source projects. You

There are probably a few effective approaches. One is to get another degree. That looks like a career reset. The easiest is probably Open Source projects. You could write the sorts of things you are interested in, even if it has been done before. For example, you could write an emulator for a calculator or for an old 8-bit home computer. You could write something to transform executables, for example from i386 to x86_64. You could create part of a valgrind clone, just doing the JIT or even a simple interpreter. You could write a compiler. You could port a compiler to output for a different architecture, or port an OS to run on a different architecture. For example, I think there are Open Source RTOSes that do not yet run on RISC-V. If that isn't true, there are so many other architectures to choose from. Doing well on the pwnable.kr site without cheating is good. There are public write-ups available, so you'd have to demonstrate that you actually understand things on your own. Getting near the top ranking ("front page is pretty respectable" according to a coworker) would be good.