After eighteen months of use, I can safely claim how useful a a developer diary is, and why you should start using one if you don't already.
Using a single file grew too large as expected. Instead each grouping is allocated an individual file. While some of these are small, the ability to navigate easily is not lost. The simplest thing to do here is just point a browser at the directory. Navigation and searching is then built in. Each week I actively split the main file file into separate files or merge to existing content.
I'm increasingly finding regular needs and uses to refer back to notes. In some cases it is quicker than searching online, simply hit the bookmark and start typing.
Adding notes works great when reading books or watching videos. All content recorded ends up in the same location.
Keeping a developer diary is without doubt one of the most useful changes to my development process. Encouragingly others are doing similar things too, but under slightly different names.