I’m pretty happy how using Agenda as a Zettelkasten is working out. It may not have the bells and whistles of dedicated apps, but the big plus for me is that I don’t have to decide if an idea belongs in Agenda or Roam or wherever.
Basically, I’ve created a project called Ideas and each note/Zettel has one idea. I’m broadly following the structure recommended for Zettel contents, adding links between notes and tags.
However, I sometimes create Zettel notes in client-related and other projects if they also belong to that project. Because of links and tags they are still part of my Zettelkasten, even if they aren’t in the Ideas project.
I’m experimenting with a project “bibliography” where one note = one article. I include link to original and make my notes in the article here. I make the author’s name a person tag so I can search and also see other items in Related Notes. I can then link from an Idea note/Zettel to the relevant bibliographic Note. Obviously this won’t work if you need to insert properly formatted references in your writing, but don’t.