So glad to see the Obamas' endorsement. And I have no problem with their timing.
IMHO, when it comes to politicians including ex-POTUSes (ex-POTI?) giving endorsements, it's all going to be carefully timed for maximum effect and/or publicity. Releasing statements one by one allows each one to reach their intended audience, whereas a 'waterfall' of announcements all at once will get them 'lost' in each other.
I think celebrity endorsements can come when they will. They mean less and less these days as far as getting votes for their chosen candidates. Except perhaps for Taylor Swift. She does seem to have a huge effect on her fan demographic. I just don't know how many of them are 18+.
I really don't think we can count on a huge drop in the Trump numbers. The people who would admit voting for him to pollsters aren't going to ever switch to Kamala. What we need is a rise in her numbers from Independents and disaffected Repugs. As well as the Dems who weren't committed to voting for Joe.