I'm looking up Thorium right now, I initially assumed you were referring to the homonymous browser (also their website feels organized a bit weirdly to me). See

I'm looking up Thorium right now, I initially assumed you were referring to the homonymous browser (also their website feels organized a bit weirdly to me). Seems a solid option for epubs, admittedly the builtin calibre reader is a bit lackluster.