Hi everyone!
I wanted to update you on my decision.
First, thank you for all the recommendations. In the end I decided that I will sacrifice note taking ability and Android flexibility for a simpler approach and went with a very open (it seems) device.
I got a Pocketbook Inkpad 3.
I'm happy that:
a) it doesn't want any kind of registration
b) seems very hackable (I was able to remove apps from the menu just by renaming some files)
c) seems to respect the connectivity preference as it always prompts if you try to use something that needs a connection, and it doesn't have bluetooth
d) works suprisingly OK with a large (300 pages) PDF with images (even better with KOReader's reflow)
e) added KOReader by just pasting a folder (OK, two folders) onto it
f) has a SD card
- KOReader has a note taking app, but the keyboard lags like crazy (but doesn't miss any letters) - good enough for my needs, with some patience

- The included "drawing" app is a joke, but I knew that it will be like that, thanks to you
Got it new for ~200€
Hope this helps someone else.
Thanks again for your suggestions!