@JSWolf, These are just personal opinions/observations ...
CoolReader is very configurable and can bring some kind of respectability to a library full of horribly formatted epubs. If you have an Android device you can easily try it for free. However, I'm guessing that your library isn't like this because you've already fixed the source epubs to suit your tastes. If rendering an epub like your Sony would have rendered it is important I think the Kobo standard ADE renderer is likely to suit you better - fixing the epub css predictably fixes the epub rendering for me. BTW I don't believe the Kobo font thickening would be used inside KOReader, but I'm happy to be told otherwise.
On the other hand, KOReader is much better for PDFs than the standard Kobo app is, but still not as good as a medium-sized tablet with a decent PDF app would be. The Kobo standard PDF reader trails a long way behind what I experienced on my Sony eink devices.
Full disclosure, it's about 18 months since I tried KOReader and things may have changed a lot since then. At that time I did get problems with my Kobo database getting corrupted. It happened each time I loaded/configured KOReader then connected to calibre. It's possible that my problem arose because I didn't take the precaution of storing all my KOReader books in a 'hidden directory' so that the Kobo apps couldn't see them. In the end I decided not to pursue it because I preferred the Kobo apps for epub and I didn't read enough PDFs to make further experiments worthwhile - I haven't had a database corruption since.
Despite the problems I had, I still enjoyed the experiment and wouldn't want to discourage anyone from trying for themselves. That's half the fun in buying new readers
