Linkedin is verging on unusable/not fit for purpose...

Example, how can I see job adverts with very very niche requirements in quite unusual roles (one of which I do actually know about eg data 'migration projects for large companies that specialize in peristaltic pumps').

I‘ve seen a job advert for that kind of thing with literally hundreds of applicants. I would be surprised, given the fact that is a niche area that only a handful of companies even operate in, and companies very rarely migrate to new systems, if there are even hundreds of people in the world who have even done something like that! That is terrible for both the recruiter and the applicant. How are you even supposed to demonstrate you have done that kind of thing before in a literal crowd of people?!