I have probably purchased more Fires and Kindles here over the years than most folks. For reading books, I ended up putting them all on my back-lit Kindles. Less distractions, etc. Just got a $29.95 4 GB one here for my wife and she likes it. Lighter weight, easy to read, good battery life.
My ancient Kindles, reflective with buttons, even do text to speech…not too great but OK. And they play MP3 files.
I find very little difference between my trendy MANGA model, my 64 GB one and the cheaper back-lit ones. All the text is easy to read. Using Calibre I can add books from everywhere and Amazon.
Which one is for manga??
As you mention you like reading sites so you may be familiar to this: https://www.gutenberg.org/
The site has been available free a long time.
My sister has kept a kindle-device in hand for years. She really has worn out a few!
A manga model was offered a couple of years ago. I think they were sold outside the USA.