Don't look on a tablet as a replacement for the Kindle reader. Most come with reflective screens that are next to useless in bright light outdoors and are no competition for a proper e reader. I have a Nexus 7 which I basically use in the way that your wife probably will and it is just great for that. If her use of storage is very little then the 8GB will do but you need to know that the operating system takes quite a large chunk of that so I would go with a minimum of 16GB or choose a tablet that is expandable with SD cards. The big advantage of the 7" tablets is that they slip into the inside pocket of a jacket so you don't have to carry a separate bag or lose the use of one hand. I'm quite happy with the Nexus 7 and the Android operating system but would recommend that your wife tries a few different tablets if possible.
OnePlus 5T review: Hands-on