Kobos let you install custom book-reading software without too much hassle. KOReader[0] is the most popular one. I like it because it allows for a ton of customization, can properly render pretty much any EPUB you throw at it (much better than stock), and can reflow PDFs (using K2pdfopt[1] under the hood). But there are some other alternatives, including Plato[2] (simple, easy to use, fewer features) and PBChess[3] (closed source, play chess on your Kobo).

The MobileRead forums[4] are quite active, you'll find a lot more there.

[0] https://github.com/koreader/koreader [1] https://www.willus.com/k2pdfopt/ [2] https://github.com/baskerville/plato [3] http://pbchess.vlasovsoft.net/en/ [4] https://www.mobileread.com/forums/forumdisplay.php?f=223