A Kindle Fire HD8 sounds like it might be a match for you. Currently $79. Go on sale all the time. Price ranges from $59 to $99 usually (I paid $59). Uses Amazons own app store by default, but you can easily add Google Play to the tablet and gets apps from that larger repository.
Note: This is a tablet, not an eReader. And you already have a Galaxy Tab S2. Are you looking specifically for an eReader? These will be smaller and lighter, but less functional for your stated needs. A rooted eReader might work for you, might not. It will be a lot more work, with inferior end results. Unless your absolute most important requirement is that the device be super light in weight. I personally prefer an eReader over a tablet for
reading books, but you laid down a lot of requirements beyond
reading for your next device. I think you might do better with a tablet (but you already have one???)
https://www.amazon.com/All-New-Fire-.../dp/B0794RHPZD
The 7" version of the Kindle Fire is even cheaper, but I'd go for the 8" because it adds not only screen size, but better resolution, bigger battery, more memory, more powerful CPU, etc. And the 8" is typically only $10 to $20 more than the 7".
It's not an iPad, but then, it almost costs less than a just the Apple branded charging cord for an iPad!