epd-wm VS wlroots

Overall if you want a DIY e-reader, your best bet is to pick a low power board that supports Linux, run the most barebones system you can on it, and reduce its power usage by aggressive means (underclocking, undervolting, "deep sleep" by turning it off after X minutes of inactivity). Then for the eink part, use a barebones desktop environment, combined with a window manager meant for eink, such as epd-wm. After that, slap an ebook reader software like Foliate on top, and you're good to go.

I'd agree. I think a lot of single device re-purposing can be useful to others, but a development platform typically indicates a more general purpose. Something like https://github.com/dj311/epd-wm but a lot of these are only for specific chips. This one seems to have more support: https://github.com/vroland/epdiy

>barrier has been unmaintained for a long time.

As if not having new features added for 2 years makes it stop working? barrier is perfectly fine on normal linux desktop installs. I actually use synergy 1.x personally which has been "unmaintained" for much, much longer. Except synergy 1.x will compile and run on anything from windows 98 to ubuntu 5 to debian 12. You can't get a waynergy or inputleap to compile on an OS more than 2 years old. And even then, as you say, it's crapshoot if the particular wayland will have libei; many like sway are actively hostile to it and never will: https://github.com/swaywm/wlroots/issues/2378

It looks like they actually patched it to filter those modes out, so presumably it worked out of the box and was considered undesirable: https://github.com/swaywm/wlroots/issues/3038

I haven't experienced any of those. The video game performance hit may be due to vsync, but I don't play games so I haven't noticed.

You can however use tinywl. It is an example Wayland compositor that can't do more than displaying one application.

Sway/wlroots has implemented this, but I can't find any discussion for KDE.

wlroots is archived on github. Is it abandoned? Just saying, that only means they moved git hosts :-D

A lot of compositors are based no the wlroots lirbary. So they are still sharing the development effort and have a common base, its just in the form of a library rather than a display server.

gamescope use wlroots