If you can at all afford it, and really like using a tablet and would rather not give up on its extra abilities again, I'd also suggest looking into a refurbished or used-in-good-condition Kindle in addition.
Either the K3 (Kindle Keyboard) or the Kindle Basic (no touch screen, no frontlight) should do very well for outdoors reading as well as reading under normal lighting (anywhere where you could read a paperback without an extra light), and keep the tablet for reading where you need the extra light - and just learn to deal with the extra temptations of email checking, web browsing, etc at those times.
If an extra Kindle, even a used one, is really out of reach, and you
can read on a tablet (because for some people it's not just the distractions that are the problem but also just reading on a backlit LCD screen in general - that doesn't seem to be the case for you though!) then .... just learn to deal with the distractions.
(Easier said than done, I know - I've learned to put my iPad away from arm's reach when I sit down to read with my Kindle, or it'll be just as bad, reaching for the tablet too often to compulsively just "check on things". Sigh.)