I haven’t used Obsidian / Roam / many of those tools. I do use The Archive for my own long-term notes / knowledgebase, and have read some about zettelkasten.
My experience with it is that because the content of your notes archive is theoretically infinite, externally-imposed structure creates unhelpful constraints. At a simple level, this is the tags-vs-folders debate. It’s good to have a place to put a note… until you want to put it in multiple places, in which case you should tag it.
That’s why the zettelkasten approach doesn’t involve filing things in folders, and even tags are just sort of a first approach to organization. A higher form of organization is notes that link to other notes, where these higher-level notes have their own structure.
You can do the same sort of thing in Agenda with links. Say I have this project / note structure:
I can create multiple views of this note system, like this:
The difference is that Agenda requires you to file your notes in a category and project, and zettelkasten style tools don’t let you file notes at all (although I think some like obsidian may have folders? not sure).
Anyway, I don’t know if that’s helpful to you. I’m just thinking out loud a bit. I am personally interested in the outcomes from different UX decisions, such as imposing structure like Agenda does, vs having no structure like The Archive. You can approximate the “no structure” approach in Agenda by using a single project, as you’ve done. And you can apply the zettelkasten tagging / linking organization principle in Agenda as well, as you’ve done with tagging and I’ve demonstrated with linking. So at that point it’s a matter of how much friction is involved in applying those principles.
Personally I keep actionable material in Agenda, and long-term material in The Archive. I have a bunch of meetings throughout the week to prepare for and reflect on. I also (to my surprise!) find a lot of value in date-based notes, where I have a plan for the month / week / day. I don’t think anything comes close to Agenda in terms of organizing notes that way. With the links I can create secondary structures when I want to (though they always feel subordinate to the main structure - I think because Agenda doesn’t have multiple windows / tabs). For my sprawling notes archive though, The Archive mostly provides the functionality I want with the least amount of friction.