This personal knowledge management system + forum concept is really forward thinking and potentially very powerful. It’s not “new” (e.g. see “Bliki”, from 2003) but it doesn’t have to be. The context is new, and therefore so is the idea. The PKM market is growing really fast because everyone wants something kind of like what you describe, but it doesn’t exist. That being said, I don’t think wikilinks would be the correct solution; “link to highlight” features being built into Discourse (as an improvement/variant of the current Quote functionality) would. The “Share” button that pops up when you highlight text within a post should provide a URL as well as an option to create a new topic within the forum using the quote (the latter feature is hidden three clicks away, and doesn’t quite work right). But I can see how it might be useful to create wikilinks to topics that don’t yet exist, which then become wiki posts upon someone clicking through and creating it.
I think your Garden is a terrific proof of concept, and largely limited by being hacked together from Discourse (do you think it would work better as a wiki, a zettelkasten, or a Circle/Notion instance?). A shared PKM can easily fall apart if post quality isn’t tightly controlled: deep-but-useless content can become entrenched without a way to discern the quality of the content before clicking through. Forums handle open-ended collaborative knowledge production much better than conventional PKMs. Here’s an interesting intermediary example: LessWrong has a “community blog” system, which is actually a Reddit fork, and it works brilliantly for their purposes. It allows them to do away with the challenge of requiring contributors to all be good at what they do from the start; user contributions (posts and playlist-like collections of posts) are not canonical (contrast with how there’s usually only one article per topic on a wiki).