I've always liked the Windows File Explorer tree directory structure. At one point I tried creating a GTD system in File Explorer using Notepad. It turned out that I was repelled. After entering text, the page looked crowded and ugly to me. I could hardly understand my typed words because I was so disturbed by the appearance of the page. I did not realize I would react that way.
I used to use an HTML text editor. I don't remember the name of that small program. I had enjoyed that, and I thought it would be a similar experience, but it was much different. In the HTML text editor, I could make the text different colors in order to indicate the function of the text (a comment was green). Text could be spaced in a way that made sense to me. I could keep my HTML orderly with the spacing, so I understood it. An HTML text editor with minor additional formatting capabilities might be helpful, if you do decide to use a text editor. It is still plain text.
People here have mentioned Obsidian, which supports markup. I believe these are essentially text files that use text to indicate formatting. As I recall, markup is only one option for Obsidian. Plain text might be another option. I believe that Obsidian can stand alone on a local computer. It is available for free with no charge for the full version. What people seem to like about it is that they can create an astonishing information web with cross-linking. There are also a huge number of plug-ins that automate functions. I believe they are contributed by the community, and some are excellent. People recommend ones that work well for them. This can create great functionality. I've never used it, but it sounds good.
I once had a friend who had kept a daily log in MS Word for years. He kept track of each day by date. It was one long page/file that he could easily scroll through or search to find any past date or actvity or person. He used the usual .doc files at that time. I do not recall that it was slowed down in scrolling or searching as time passed, and it got longer. MS Word might have the capability you need. I believe Word files can be saved as .txt files. Maybe that could be made the default. You can create hyperlinks in Word, which might help navigate your one-page system.
I have read many times in the GTD book and heard in videos that it really doesn't matter what tool you use for your GTD system, whether it is paper or digital or hybrid, since it is just a bunch of lists that can be created using any tool. Your system will likely continue evolving as you use it. It is the best practices we make habitual that create our GTD system. What matters is that you feel attracted to your system because your tool is fun to use. Then you will likely continue to maintain your GTD system.
You might want to remember the GTD setup guides.
https://store.gettingthingsdone.com/product/trello-setup-guide/
