Frank, in case you haven’t come across it yet, there is a stub page on the IndieWeb wiki about using our websites as digital commonplace books. Hopefully it will have some useful information, articles, and examples for you to use as you continue hacking. Feel free to add your own thoughts to it as you experiment.
While I do like the way that WordPress makes it easy for one to create link previews by simply putting a URL into the editor (as in your example), I’ve generally shied away from it as it relies on oEmbed and doesn’t necessarily put the actual text into your site. (Not all websites will provide this oEmbed functionality either.) I mention this because a lot of the benefit of having a commonplace is the ability to easily search it. If your post only has a title and a URL, without careful tagging it may be much harder to come back and discover what you were searching for later.
I’ve started an article on how I’m using my website as one, but still have a way to go before I finish it. A big portion of my workflow relies on the Post Kinds Plugin and its available bookmarklet functionality. There are also a lot of nice Micropub clients like Omnibear that making bookmarking things quick and easy too.
In the erstwhile, I ‘ll note that on my own site, I tag things relating to my own commonplace (thinking about and building it) as “commonplace book” and for examples of other peoples’ commonplaces, I usually use the plural tag “commonplace books“. These may also give you some ideas.
With respect to the Medium article which you linked, I’ve seen a recurring theme among bloggers (and writers in general) who indicate that they use their websites as “thought spaces”. Others may use similar or related phraseology (like “thinking out loud”) but this seems to be the most common in my experience. Toward that end, I’ve been bookmarking those articles that I’ve read with the tag “thought spaces“. Some of those notes and websites may also give you some ideas related to having and maintaining an online commonplace book.