The PKM space for neovim plugins is a surprisingly deep rabbithole but it can take quite a while to find what you need.

For completions and tagging https://github.com/Feel-ix-343/markdown-oxide works well for me and does pretty much everything I need.

There are also a glut of all-in-one solutions like https://github.com/nvim-neorg/neorg (org-mode for neovim) and https://github.com/epwalsh/obsidian.nvim.

Graphing is a bit harder because it doesn't intuitively match up with nvim's interface, but I'm pretty sure I saw something on Reddit the other day.