Milkdown is a ProseMirror based editor that integrates Yjs as the collaborative editing solution. It's a plugin driven WYSIWYG markdown editor.
{% embed url="https://milkdown.dev/" %} Milkdown Homepage {% endembed %}
They have a great getting-started guide for collaborative editing with Yjs & Milkdown:
{% embed url="https://milkdown.dev/docs/guide/collaborative-editing" %} Getting Started Guide {% endembed %}