I'd suggest cheap Android-based Chinese e-Ink e-readers if you want flexibility. My current one is a Bigme B6, which was for sale in my country a few months ago.

Their main advantage is providing access to all e-reading apps available on the Google Play Store, including Amazon's own Kindle app, as well as sideloaded ones such as KOReader.

On the downside, the battery life on those isn't as good as that of dedicated Kindles, Kobos, or other lightweight e-readers, but they still hold a charge for four or five days if one turns off their antennas, which is plenty of time to recharge them.

As for the ebooks themselves, I switched to purchasing from Kobo and other ebook stores. Some sell DRM-less ePubs, which is nice, while those that come with DRM can be easily liberated. And for the occasional Kindle-exclusive that is struck with (temporarily) unbreakable DRM, the Kindle app, although annoying, works well enough.