I also make my ePubs a free download off my website which prevents me as an author enrolling in this. So just about anything on Kindle Unlimited is only for their ecosystem.

There's self published books and stories on other platforms. There's a bunch that are free too. There's some author's that only use Kindle, but there's plenty of independent stuff out there not Amazon.

Might depend on what you like to read. I haven't hit this a lot. There was one title my daughter wanted which was Kindle exclusive in the US but I was able to get as an Adobe epub from a European seller.

Similar with “only on Audible” audio books. Not a majority yet I don’t think, but it’s a big chunk of books restricted to Amazon platform.

Honestly, I think the Kindle Unlimited (KU) program should be persecuted as an illegal monopoly. A lot of authors actually don't actually like it, but they have to do it because it is the only way to have viable income. This tends to happen with more niche genres though. As an example almost all books in the harem-lit (1 guy and a bunch of lovers) genre are tied to Kindle unlimited. Meanwhile, this creates a self-fulfilling prophecy as pushing more readers to KU and thus making alternative methods of selling even more difficult. Though since KU forces exclusivity on only being sold through Amazon, the authors are forced to switch to it.

I could probably go on a similar rant with Audible too, but that is different story. In short, Amazon has way too much influence over the entire publishing industry.