There's an open-source WYSIWYG Markdown editor named Zettlr, the author of which is very fond of the Zettelkasten method and seems to provide some features meant to help with it:
Wow that looks really neat! Too bad that there seems to be no Webclient, although it seems to be a Webapp (based on Electron) ... Last weekend I hacked a tiny WYSIWG flat-file HTML wiki for documenting a small hobby project with a non-coder friend, but now Zettlr looks like all I ever wished for, for that purpose. Except I would've to bug my buddy to install the Zettlr applciation as well.
As I hinted to in the comment, if you just want a WYSIWYG Markdown editor, and you're not particularly interested in the Zettelkasten aspect, take a look at https://marktext.app too, to compare which one you prefer. In my personal opinion, Marktext seems in particular more robust.
https://www.zettlr.com/
I don't know much about Zettelkasten, but I used this app as a Markdown editor of choice for some time until I discovered Marktext.