(...oh, now I get it: <https://remarkable.com/>)
I hope it will soon be the best e-reader on the market, thanks to KOReader[1].
[1] https://old.reddit.com/r/RemarkableTablet/comments/jsb3r7/ko...
A feature that does exist - and I maybe should have mentioned, is that one of my house mates finds it really useful for remote teaching, they screen share the remarkables screen to their students and use it as something like a blackboard.
If you expect more from your ebook reader than that, it is not just because of software.
As a sibling comment mentions that might be fixed soon. V1 had long since had support for third party apps. V2 is taking a bit of time (because controlling eink screens in weird) but will probably have third party apps support soon. At that point you will be able to use one of the well known open source ebook reading apps.
Edit: And to be clear, all the third party app support is unofficial, the help menu on the tablet includes the ssh password for root, everything past that is just reverse engineering (it's a pretty typical linux system apart from the display).
It’s great for reading long standard documents and cross reference between them and supporting material like examples etc.
Ultimately I currently use Books because I do a lot of multicolored highlighting which is far more consistent in Books, and because Books lets you set bookmarks (to go back and forth between main text and endnotes) which Acrobat inexplicably still does not support.
Ultimately every PDF app is good at some things and terrible at others, and you're just going to have to find the app that has the best set of features for you. It is pretty sad that all the apps have significant drawbacks for certain common workflows.
I started off my "learning more than what you get taught in school" reading PDFs on a tiny phone, I now do it on a bigger phone, a big DPI monitor etc. none of these platforms makes a blind bit of difference to me because my eyes aren't the bottleneck.