[JawadLeLogeur]

The first thing to remember is that tablets are far better than any e-reader for PDFs. Of course, the e-ink screen makes reading easier. So the first step is to check if you can read PDFs for a long time on a tablet without feeling dizzy or irritated. If yes, just get a Galaxy Tab or something or check out the Lenovo Yoga Book which is very weird and misses many of your points but might actually interest you.

With your criteria in mind the Onyx Boox Note could be a good fit. Has android with a bunch of specialized apps for e-ink. It misses 5 and 9 and is over 500 dollarydoos but lower than 700. There is the Note+ which is very similar but with a flush glass screen. The Note S is smaller and has worse specs, is quite cheaper, but has a very bad resolution which kind of defeats the purpose of that bigger screen. However you should make up your own mind with video reviews. The Note Lite is like the Note except it doesn't support stylus input and is cheaper.

The Remarkable is high quality but expensive and has very limited software. It is however designed with scribbling and annotating in mind. It's kind of like the Macbook to the Onyx's PC.

You can get a cheaper 7.8" device like the Likebook Mars but you'll have to get your own stylus. According to reviews it can support PDF annotation and has Google Play anyway so should be fine. The Onyx Boox Nova might drop in a little while too. Kobo is releasing a new device this month (presumably also in the 7.8" range) so perhaps wait a few weeks.