Trying to get away from my phone more, and an e reader sounds like a good way to get myself to read more. Kobo is sounding like the most recommended one based on some googling, but I’m also ok with jailbreaking something like an older kindle from eBay. Something that plays nice with Libby would be ideal but not required. 
what are the upsides of koreader?
Better font rendering, more options for customizing regular book formatting and fixing bad book formatting. A plugin ecosystem.
In addition to the other comment, the PDF reflow function is great. No other ereader software compares. Its not perfect, but its still really useful when I can’t get an epub version of something.
Also Idk if kobo has the function built in, but I use koreader to connect to my self hosted calibre opds catalog to download books to it.