I use a Nexus 9 and an anti-glare screen which makes the screen nice and matt and glare free. The only downside is battery life is not as good as an e-reader, but it is much more flexible.
it can run Kindle but I use Bookari Premiun for non-kindle sourced books, and it is prefereable as it has a night mode (inverts colors) and a crop mode so I can reduce the useless borders on PDFs to make them more readable.
The Kindle app used to have a mode where you could invert the colors easily for night reading, but it seems to have gone.
I also run an app called Screen Filter which cuts the light output even more for nighttime reading.
I am happy with it – read lots of technical books on it.
Used to use an ereader but not so good for diagrams and size was an issue as well.
The Samsung Tab S2/S3 tablets have a nice aspect ratio – avoid 16:9 tablets. Used to have a Pixel C but it broke and got a refund as no replacements are available.
And look for a tablet which has good anti-glare screen covers available.
This one was good for the Pixel C :
this ocmpany were good for my Nexus 10 and 9 :