I really think you are missing the boat by dismissing Kobo for what seems to be no good reason (at least based on what has been posted so far).
If you use Calibre and DRM removal tools, no device is locked in to any vendor.
Check out the Kobo H2O. From your list, it fulfills all of your Must Haves and most of the rest:
- HD resolution
- touchscreen
- built-in light with blue filter
- epub
- 6 inch screen or 6,8 at most
- price range of €100 - €170
- Waterproof
- ability to load custom dictionaries
- open system
- Wifi
The only ones missing are:
- New Oxford English dictionary
- physical buttons
- Cloud
- android
And cloud would be on that list if you buy books from Kobo.
If you drop Waterproofing, you could look at the Clara HD, which otherwise meets all the same requirements.