I haven't changed my output format since I got my paperwhite a few years ago, and everything still looks great on my Glo HD and Aura One. I should probably look into that. On a side-note, you should install the
Kobo Touch Extended plugin. Save the books in your Calibre library as epubs. When you use Calibre to send them to your reader, they will be automatically converted to kepub (Kobo epub) during the transfer.