I would consider myself pretty close to an expert with JavaScript. I've been using it for 5 years professionally, currently use ES2020. Fairly familiar with Node 12, V8, etc. But I'm always worried about those situations where you dont know what you dont know. This might sound strange, but would anyone recommend any reading material or blogs for someone at my level?
read "you don't know JS" series of books, also second edition https://github.com/getify/You-Dont-Know-JS I find the workshops very helpful as well, which I've watched through my oreilly membership don't know if you can find them elsewhere, I would suggest combine this with reading the specs.
on edit: one reason the specs are not enough is generally specs will tell you what needs to be implemented but a good book, like these ones, will tell you how it has been implemented or what the spec implies for implementation and what all that will mean for you as a user of the language.