I've been using getoutline.com as my note-taking app. It's intended as a team knowledge base for documentation, but I find it works well even as a single user

I've been using getoutline.com as my note-taking app. It's intended as a team knowledge base for documentation, but I find it works well even as a single user for notetaking. It's open-source, and self-hostable, and integrates with draw.io and mermaid graphs. And everything saves as markdown, so it has good portability.