I'm also interested in this, but I'm more interested in what you need to say and do to convince other people you're an expert.
> everything else frontend is pretty much resolved
Has it? The technologies will change again. Pretty soon, if I have to guess. What's valuable here are the concepts that are universally applicable and have been since it was called client programming in the 70s: state management, rendering, API integrations, architecture.
You should be able to offer solutions to problems impromptu. You should be dependeable in a wider-group meeting involving business or other competencies to represent what does the frontend say about it.
This means having knowledge and experience to offer solutions without googling.
Beyond that, you also need to be on top of the latest (not the newest) standards to follow. You need to stay up-to-date and filter through the waterfall of sh*t trends in the industry, to know what's actually relevant.
While doing all of that, you need to maintain your past/fundamental knowledge...nitty gritty details, to help your colleagues.
You need to do all of this while (most likely) being representative spokesperson/front for a frontend project at your day job.
> everything else frontend is pretty much resolved
Has it? The technologies will change again. Pretty soon, if I have to guess. What's valuable here are the concepts that are universally applicable and have been since it was called client programming in the 70s: state management, rendering, API integrations, architecture.