This is not a suggestion, it’s probably fairly stupid, but it’s what I’ve been using.

I’ve been using a convertible ThinkPad L390 Yoga as eBook reader as well. I never considered a 2-in-1 laptop, but it was cheap and I heard the Yoga versions have better colors (display). I thought I’d never actually use it in tablet mode, that my touchscreen would be unused, free of smudges. Hell, I didn’t know what I was missing, it’s awesome.
I’ve been using it to read eBooks, in portrait orientation as a tablet.

Software wise, Arch Linux (btw), KDE Plasma 6, Arianna eBook program.
Not optimal to be honest, Plasma 6 has some annoying bugs, and Arianna is broken as of recently. I suspect some depency issue, but anyway, for the time being I use the Flatpak Arianna package.

But I do like the experience. If I need to check some word in dictionary I can do it on the same device. Plasma 6 has touchscreen gestures, for example I use sliding from right to switch between windows. So, Arianna and Firefox with Wiktionary open at once, reading the book, unknown word, long press it, copy, slide from right, Firefox window, paste into Wiktionary, boom!
And to save extra power I use Bluetooth for network connection rather than WiFi. 1Mbps is plenty for dictionary searches.

Oh, important to me, when turned around there’s a deactivated keyboard on the other side that I can fidget with while reading. I feel like it helps keep me from getting distracted by something else. Just mashing the keys with my right hand fingers and clicking the trackpad with left.

Disadvantages of this:
Hardware wise, it’s a 1.5kg 13.3 inch eBook, so… perhaps not your glass of water (I don’t drink coffee ¯⁠\⁠_⁠(⁠ツ⁠)⁠_⁠/⁠¯).

(No need to read further, this is specific to software I run on it.)

Software wise, well, you can choose different software, but bugs. Visual glitches like the taskbar switching to floating when using virtual keyboard or the window occasionally staying retracted from where the keyboard was (fixed by toggling affected window out and back into fullscreen) are okay.
What’s worse, inactive window translucency can get stuck, i.e.: if the window gets stuck translucent even in foreground, and you close it, it’s now permanently on screen as ghost window and you’ll have to log out and log in again.
Worst, toggling Bluetooth (usually when done quickly after log in) may crash the system partially. The GUI completely freezes, tty works, but reboot won’t fully work. It will get stuck mid-way, so I recommend logging in as root, enabling magic sysrq (echo 1 > /proc/sys/kernel/sysrq), issuing reboot, let it freeze, then Alt+SysRq+REISUO (one by one while holding down Alt and SysRq keys) to shutdown.

(Bluetooth service cannot be stopped or killed, nor plasmashell)

P.S.: Use Wayland with touchscreens. X11 has no touchscreen support, it just emulates a mouse pointer which is suboptimal.