Regarding niche, I agree with most of your points, but I don’t agree that choosing a niche is necessary to establish yourself as an expert. You compare a niche specialist with a generalist, with the assumption that the generalist isn’t as much of an expert because they do too many things. But that’s a very false generalisation. I would consider myself deep generalist. I have a varied background that includes a lot of general design work, but one or two areas of deep specialisation within that. You’ve obviously heard of the “T” or “M” shaped professional. In my experience, a T shaped deep generalist actually offers more value than someone who is overly specialised at the cost of not having a wide enough set of complementary skills to support their expertise and add more value to their offering. Even more so if they are a freelancer.
Where I find the value in niching down is in the way it allows you to focus your marketing and make it more personal. Yet that’s simply a marketing channel. You could have 10 of those for different niches. It doesn’t actually require you to restrict your work to a single niche.