There's nothing like a dedicated e-reader like a Kindle for reading. I think your question is very difficult to answer - it depends on many factors. Good luck.
I don't have the best of eyes and I read Kindle books on my G3 every day without issue. In fact I used to use a 4.7 in phone with a much worse screen and still had no issues. Don't forget that you can change the text size anytime you want. I have had a couple of issues with newly downloaded books getting corrupted but it was my fault for closing the Kindle app too soon (like maybe 30 seconds too soon) before the book had a chance to fully download.
I use my G3 everyday for reading...it works great with the kindle app as you can adjust everything... The text is so nice and clear because of the 535 ppi..you won't have a problem..