I Personally use kobo but i have used Thalia or Hugendubel in the past aa well. I think you can use nearly every bookstore and transfer them with calibre.
But sometimes you'll get a asc(?) file instead of a epub and then use some software to "convert" it and maybe remove the drm
While i agree with you that a custom os would be really nice, i think the term jailbreak is appropriate since jailbreaking other things like an iphone is also for installing other software not from the manufacturer. But I'm no expert...
Amazon disabled the the possibility to download the books you bougth onto your pc and sometimes they change or delete books wich will change the books on your kindle.
In the yt video linked the person says the changed for example the cover of books to advertise a new show based on the book.
I've done the same (except the 🏴☠️) but after reading the the koreader docs i found it gives you just so much more. For example i can see my calibre library from koreader and download any book i own withiut using my pc. And because ive it on an server i can do it from everywhere.
Every time i see a post like this i think this one must be from Jessica.... It never is :/