Letby would not have had a defence team until late 2020, and I doubt that her expert witnesses were assembled before 2021 at soonest. She would have had solicitors present when questioned after her earlier arrests (I hope) but she was only charged in November 2020.
I don't think it would have helped much, in preparing the defence, that Letby had had these issues hanging over her since 2015 or 2016, because she had no way of explaining events and preparing a defence. The prosecution hypothesised invisible crimes at any convenient time. How could she defend against that, and how would she know what caused the deaths, which is a matter for pathologists and medical experts?
Once charges were announced, experts could not come forward in the press. They had no reason to at first anyway. They didn't know what the prosecution would be claiming. During and after the trial, up to the end of the retrial, various experts did contact lawyers and politicians to express their concerns, but they could not do so in public. That would be contempt of court, and publication / broadcast would be blocked.
That is why you might have the impression that experts all appeared suddenly in a short time. They were unable to speak out in public in the UK until about this time last year. Journalists were sitting on stories. The New Yorker article could not be accessed online from the UK.
You see that your impression of events comes from the fact that the media and public figures operated under restrictions while the legal process ran its course. Of course there are good reasons for that. But it is all the more important then that the media and experts are able to comment and critique once we complete the legal processes, and that we don't wait passively for justice to happen in the dark. To cast light on these issues is not a media circus.