The final book in the Foundation series isn’t really about the Foundation at all. As I’ve re-read this series the more I’ve become convinced that Asimov really ran out of ideas with the series. It started on the idea of a Foundation designed in the final days of a Galactic Empire to overcome 30,000 years of ruin in just 1000 years. The Foundation overcomes a series of crises as it progresses – that’s the first book.
Then Asimov introduces a mutant who can control emotions on a massive scale and then to get out of that pickle creates a Second Foundation of mentalists who defeat the mutant. But the creation of the Second Foundation is another creative dead-end and so Asimov creates Golan Trevize who is always right but doesn’t know why and then Gaia a planetary organism that has a perfectly balanced ecology and mental powers. Asimov has Trevize choose a future from neither the 1st nor 2nd Foundation and instead choose Gaia and Galaxia – which is the last book because Trevize doesn’t know why he made that choice but thinks the answer is on Earth the planet or origins.
So Foundations don’t appear but we do have hermaphrodites, robots, religious people who love freaky sex and lots of and lots of clunky conversation.
The series I think became progressively weaker the further it moved away from its original premise and that happened because Asimov (I think) wrote himself into a creative corner he could never truly get out of.