Most formats aren't going to stand the test of time and unless you want to spend a lot of time converting from one format to another its usually best to stick as close to just text as possible. Initially I used simple ascii text documents in a folder structure which I maintained backups of and then gradually I have used more markdown and use Dokuwiki currently for the software to display and edit. It still works from files so it is still just text but it links them together and offers some benefits beyond just text but it is still completely readable and will be in 20 years time long after I dump that software and use something else.
Whatever you do consider its got to last 40+ years of a career and if it becomes too difficult to maintain you will likely stop.
Yeah, my notes from online courses and things all go into org-mode documents. Still plaintext, but with the convenience of inline LaTeX math and executable code snippets. 20 years from now the in-line Python or R might not still run, but the output gets saved into the file at least.
Whatever you do consider its got to last 40+ years of a career and if it becomes too difficult to maintain you will likely stop.