Depends what you mean by "standard". It is very widely used, but the closest thing to a standard in the sense of something needed for interoperation is docx.

> standard in the sense of something needed for interoperation is docx

100% this. Word, or docx more specifically, is used for exchanging documents all over the place. It's readable (and writable) by Google Docs, Word, Pages, Open Office, and more I'm not aware of.

I do some proofreading, and I have done typesetting for a novel, and if the inputs I get are not in a Google Doc, it's in docx file.

The standard, as in what novelists enjoy writing, a software to enhance the writing experience.

Not really that either, honestly. I know/have heard a lot of writers who just use Word for that, too, Brandon Sanderson for one. Scrivener is widespread and well-loved, but it's very optional.

Somewhat. However, their manuscript → markdown export has issues. I have seen people struggling with the flavor of its markdown a bit as well.

OP has a great solution that focuses on markdown alone. And that's a welcome change!