I mostly do this, but with a physical notebook. I wouldn’t be able to work with a .txt file because it’s too limiting: you can’t draw anything, you can’t easily make arrows between stuff, you don’t have the nice mental reward when you strike some item off your list.
This is for the day-to-day organization. For the rest, I dump all my knowledge in a wiki (MediaWiki), and I use iOS/macOS’ Reminders app to remember things to do far in the future (like "cancel XYZ subscription" in 6 months) or at very specific times.
I have an iPad and use notability for this. I have a 350 page note that's filled with to-dos, doodles, screenshots of quotes, etc. I love being able to just scroll up endlessly to see what I was thinking about or working on, all in chronological order.
This is for the day-to-day organization. For the rest, I dump all my knowledge in a wiki (MediaWiki), and I use iOS/macOS’ Reminders app to remember things to do far in the future (like "cancel XYZ subscription" in 6 months) or at very specific times.