Why a laptop and not just a mobile monitor?

You can already by a 13 inch mobile e-ink monitor. There's the Onyx Boox Max series starting with the Max2 (the current model is the Max Lumi) with HDMI in and the Dasung Paperlike monitor. They are pricey, however.

What would be an advantage of that?

If you have some kind of overlay screen you can have both worlds. But I was still under the impression that E-ink was the only option. Today I learnt about transflective screens. I hope they catch on. I would buy it.

It can be used as a PC display, which I'd kill for (figuratively) as long as it's affordable. If it's $200 or more, that's not what I consider affordable.

To develop a laptop is a much bigger goal than developing a monitor.

And everyone has different taste/priorities when it comes to laptops.

I would like to put the monitor on a stand behind the laptop so it is on eye height.

Since such a screen would be most usable outdoors, I wonder about the setup. I guess if it'd be light and thin enough it could be put on top of the screen in that case? Or would you have a separate stand for it?

I would think a seperate stand. So I can put it behind the laptop and get it to eye height.

E-ink monitors already exist. Look up the Dasung e-ink monitor.

That might be an option. Have you tried them?

I have one and use it outside regularly (e.g. [0]), it's pretty amazing!

There's some open-source tooling[1] for it which can control screen features like brightness/refreshes/rendering modes/... and I've set it up[2] to work with my unprivileged user so that I can bind these features to keys in my Emacs.

With these screens you want to use light themes, and use tooling to improve contrast on websites (a friend wrote a Firefox extension[3] which is amazing for this).

[0]: https://photos.app.goo.gl/eGgGYR7ecBqv9gqy8

[1]: https://github.com/leoluk/paperlike-go

[2]: https://cl.tvl.fyi/c/depot/+/3029

[3]: https://github.com/adisbladis/einkmode-firefox