Kobo makes several very nice ereaders which support EPUB, MOBI, HTML, TXT, even CBR/CBZ out of the box. They technically support DRM through the books sold in their marketplace, but you never have to use that.
Obviously it all depends on personal reading preferences but my favorite reading devices are the backlit e-ink/e-paper devices.
- Tablets are heavy. You should be able to hold a reading device with one hand for a long time without an uncomfortable weight. - Tablets are battery hungry (even though this is usually not a big problem for me). - There's less distraction with a "reading-only" device.
I understand that you don't want to support Amazon (which rules out Kindle Paperwhite) or B&N (which rules out Nook GlowLight). In the past I used Sony PRS devices and I was very happy with them but I've never used the new ones such as the backlit PRS 700. Maybe something like PocketBook?
Have a look at this old HN discussion. There seems to be some good advice: https://news.ycombinator.com/item?id=4682793
I'm not sure about dedicated reader devices (like with ink-screen or the like).