One thing I still miss in most markdown tools is good rendering/sharing of large architecture docs and Mermaid diagrams. I ended up building my own markdown file reader - https://mdview.io which handles large diagrams/tables much better than typical note apps

I have a simple script that previews md pages as html and hosts a liveserver so it dynamically updates, renders mermaid / syntax highlighting etc. Super useful when working with an agent when planning out a piece of software. Page dynamically updates as you go, and super useful to have diagrams visible. I'm prompting a lot more to get diagrams included as part of the planning stage (or whenever).