[culi]

Well there's quite a few options. The most popular I think is also open source (Markdown for Humans). Not sure what disqualifies it as "good" to you

[engomez]

Our bar was "Notion-grade", i.e. drag and drop blocks, slash commands, select to highlight/bold, etc.

I don't think I'd seen an extension that does that. It was a technically very hard problem, rich text editors usually use a lossy intermediary format (e.g. prosemirror).