I did find it hard to get the bezels to be even and it still doesn't always format well depending on the book. I still kind of prefer the default viewer.
I use this plugin in KOReader which lets you download books from Z-Library within KOReader.
https://github.com/ZlibraryKO/zlibrary.koplugin
Another cool thing is you can use custom fonts and dictionaries. I use the OED short version and the Wikipedia one. There's built in ones that it can download but pirated ones like OED you have to upload yourself.
Unfortunately on MacOS it seems you have to use Amazon's app to access the filesystem.
Edit:
They changed it on the newer Kindle which I have.
There's open source alternatives to the send to kindle app.