I also use plain text files for a lot of my personal organization. My system isn't quite like what OP describes, but it has some things in common. Some of my files are also structured by date as a never-ending journal. This isn't for a todo list, it's for a journal of things I encounter that I'd like to be able to find again and that I don't want to accumulate as clutter elsewhere...i.e. in browser tabs, etc. Sometimes it's a web link, or maybe something I learned somewhere, something someone told me, etc. I include notes whatever words / strings I think I might use if I want to find this particular thing later. I use org mode and make each date be a top-level bullet so I can nicely leverage powerful text search tools like ripgrep, regular expressions, etc.

I don't find it useful to force everything into a single file. Instead, I'll organize these text files somewhere inside a directory structure that I can recursively grep. Unlike the OP I do use mutable TODO lists to track high level lists of things that I want to continue to spend mental energy on, but I do like the chronological list of done things and I might think about adding something like that or maybe augmenting the chronological notes file I already have.

I do depart from the world of plain text for keeping track of larger amounts of information such as good papers I encounter, complete blog posts that I might want to refer back to, etc. For this I use the fantastic DEVONthink tool. It's got a large array of powerful features including automatic OCR and indexing of images and an excellent search feature, but the one that I use the most is its ability to make a "web archive" from a link. This downloads all of a web page's resources and stores them in the database locally, making it really easy to refer back to things that I've seen before regardless of whether I have internet access or not, whether the website is still around, etc.