You should try PBImageViewer, it's more bare bone than KoReader, but it's my favorite manga reader (and it can open CBR archives).
https://www.mobileread.com/forums/sh...d.php?t=162852
I have installed firmware 6.1.1229 on my Inkpad 3 (from 5.2), it works fine, but I find the integrated manga reader sluggish, PBImageViewer and KoReader works fine though.