We use Slite extensively at work. Works pretty well to quickly create/edit documents with a nice editor. When you type Markdown, the content automatically resolves the styling/headers/lists. Multiple persons editing a document at the same time is (of course) supported. And every document can be exported as Markdown or PDF.

Sounds a lot like something like Notion?

Notion is very sluggish and feels very unlike a markdown file-based system to me.

Agreed, I like certain aspects of Notion, but as a FE dev myself, I think what they’re trying to do is too complex. I don’t see how they could ever make it performant.

Notion doesn't use Markdown.

Just type in markdown :-). It mostly works for input.

Sure, you can add asterisks around things and it will convert stuff to bold, etc, but Notion isn't actually a markdown-based app – the underlying representation of a Notion doc is stored as JSON.

Product with corporate lock-in, but Quip allows the user to use markdown in their documents, and export their documents into Markdown.

Dropbox Paper should meet your requirements. I tried switching to it a few years ago, but the problem is that the other people I'm collaborating with always want to use Google Docs, and when I'm writing a document by myself, I'd much rather just write to a local file.