For tracking in general you need a system and a medium. The bullet journal method has worked well for me for many years. The systems and structures within the bullet journal change over time, but you can jump back years, and see what you were thinking at the time. More recently I have been using GTD within the bullet journal framework. I highly suggest reading at least those two methods before building your own system.

+1 for outliners. Logseq was the first note-taking app that really got me to a daily logging habit that stuck, great for friction-less capture of random thoughts and notes, medical and what not. Flat wiki links and backlinks take some getting used to, but absolutely amazing for discovery and organization of notes.