I feel seen.

Yes, scattered around my apartment are many many once-used attempts at organizational systems. The trouble is I start trying to use one to remember things and then forget about it!

Just use a list!

Forgot the list... or where I put the list... or what list I needed...

Notes.org file in emacs (or notes.txt in vim, or whatever similar thing) with different sections for different topics. Put everything in there. Sync it to other devices with syncthing. It's searchable, it's everywhere. I think that's a lot safer than dealing with a physical notebook or planner.

Trying to edit and sync a plain text file across iOS, Linux and Windows is such an effort every dozen times I've tried doing so I've lost motivation halfway.

I don't have any iOS devices, but in my comment I recommended a specific piece of software ("Syncthing" is the actual name), and I can assure you it works well on the other operating systems you listed, as well as Android.

If you want to edit the file in multiple places, I think Syncthing is pretty much the best option there is. If you plan to do all edits at the same device but just want to view it elsewhere, something like regularly copying the file to the others with rsync or sftp or whatever your preference is may be enough. I manage some files in syncthing while others are just backed up to a server regularly with rsync. The downside of the one-way copying approach is that you may end up with an old version on some machines at some point, or if you suddenly do want/need to edit on a different device, you then have two versions of the file to deal with. Syncthing is kinda like Dropbox, but it's free software and decentralized. I think it uses bittorrent to copy things between your devices. If you consider yourself forgetful, a more automated/background solution like Syncthing also has benefits over manually copying stuff. You can just read/edit your file as normal and it'll sync so long as the daemon is running.

Just make sure you don't try to sync a large number of files (for any syncing solution), or your laptop battery life will go through the floor.

Syncthing is cool, but doesn't work on iOS. I mean, AFAIK it works but not in background, which makes it pretty useless.

Should be useful enough for a text file.

I have tens of org files scattered across different machines.

Fun trivia fact: bullet journaling was created to manage the inventor’s ADHD. I personally go back and forth on how helpful it is for me personally, but it’s been more so than many other strategies.

Now that does not surprise me at all. It's a nifty system that I didn't quite click with when I tried it, I should give it another go perhaps.