Journaling system? More like no-systems.
I used to keep a notepad next to me on the desk. It collected random thoughts throughout the day, scribbles, snippets of things I wanted to remember, idle loops and lines and shapes. Useful on the day to day, and always fun to flip through years later (why did I draw a duck eating a now-dead URL?)
This always seemed like a good way to take a load off my short term memory. I never thought of it as a system and there was no explicit goal, it's just something I did because it occurred to me to do it, and I enjoyed it. Years later, I'm starting to realize how good that practice was, and how it avoided a trap I have since stepped right into.
A few months ago I wanted to get back into keeping some kind of notebook. Instead of being smart and just doing it, I went to research journaling systems. I read about and tried bullet journaling, morning pages, prompt-based writing, micro journaling, and half a dozen others, never feeling like these simple systems were quite right for me. I also tried more structured systems that required setting up pages and index beforehand and putting different types of thoughts into different journals. That failed even faster--what if I didn't have my Work Ideas notebook when I had a work idea?
The thing about all of these systems is they made me not want to write. I love writing, have been doing it for a living for decades, and do it in my spare time. But tell me I need to write a certain way and suddenly I'm not interested. That's not me being a toddler, and it's not a case of me just needing to "buckle down and do it." No, what we have here is a fundamental ill fit to my personality. It's a method someone else designed, ripped the soul out of so it could be distilled into a 'universal' system, then fed to me one byte at a time. No thanks.
What journaling system works for me? The no-system of having a notebook and letting it collect things as they fall into my awareness. This isn't a sneaky stream of thought journal or anything like that, it's just letting the weight of a thought press against my brain until it wants to push into the external world, then making it easy for that to happen. One journal, no index or table of contents. When that journal is full, I move on to another one.
The closest thing to a system I have here is I go back and read through old pages every once in a while and copy down notes that still seem interesting. Those go into a commonplace journal, which sort of has categories (there's a section for quotes, then a section for everything else), and my commonplace book is a forever reference.
My main takeaway is to just shut up and do things more often. Don't see what other people have done, just go. Chances are I'll grope my way through and find a place that makes sense for my own tempers.