Honest question: How confident are you that you'll have access to those notes on your Remarkable/Kindle 20 years from now?
I love my Kobo. I highlight stuff with it. But for anything really important, I then go to my computer, go through each highlight, and make notes in my org file.
(I probably should automate it - I know tools exist to extract all that metadata from the Kobo).
So, this is kind of not directly related, but sort of is.
I teach an adult Bible study class, and I use Logos for all of my notes and highlights. Your question touches on one of my specific concerns, because they use a proprietary format for their datasets. The best solution I have is that I export the notes and format them in Markdown, and I distribute them to the class via one of my blogs. Obviously, I can't include verbatim copies or quotes from the sources I use, but it's better than leaving the notes inside a platform that could (potentially) disappear in 20 years.
I cite all sources used in the footnotes of every session, so at the very least, it should be possible (theoretically) to obtain the source material in printed form and find the exact page number (Logos stores page number metadata in the digital copies). It's probably not ideal, but it's the best thing I've come up with so far that doesn't require a lot of duplication of effort.
I'm personally extremely confident. Those Remarkable tablets are very hackable. I pay for their sub, but I've also whipped up my own app for syncing content between my RM2 and my home server so that I can send news articles to it, mark them up as I see fit and archive them.
The Remarkable can spin up a web server if you connect it to your computer via USB. You can then access the files through a web browser by entering a dedicated IP address, which the Remarkable will give you. This features is disabled by default for obvious security reasons, but it's trivial to enable in the Remarkable's settings.
Point being: as this feature works perfectly fine in Airplane Mode, you don't need an internet connection to use your Remarkable.
It's possible to download a marked-up PDF as a PDF from a Remarkable -- I can't speak for the others.
That's a decent enough solution if they let you automate somehow (e.g. sync to PC once a week without me thinking about it). I could then build further automations to parse those PDFs.