Foliate is a wonderful FOSS ePub reader for Linux. It seems to offer all of the customization options you listed.

https://johnfactotum.github.io/foliate/

Just downloaded it. It solves every single problem I had. Thanks!

There's BIbi which is a nice open source ePub reader ( https://github.com/satorumurmur/bibi )

What platform are you on? SumatraPDF handles most ebook formats and is nice and zippy.

Ubuntu/Linux Mint. I like to customize the typography, the colors, the text width. SumatraPDF doesn't seem to be available on Ubuntu though.

Apparently people have had success [1] with Wine, but that's presumably not ideal. It does have some customizability but probably not as much as you're looking for.

[1] https://forum.sumatrapdfreader.org/t/sumatrapdf-on-linux/135...