To do (Steam Machine edition)

Power-off fix in the live session. Without it the Steam Machine starts again right after shutting down from the live ISO. Build steamify-fremont-poweroff.ko in steamify-prepare.sh against the headers of the ISO's kernel, put it in airootfs/usr/lib/modules/<version>/extra and run depmod (small); or ship DKMS packages from a local repo (~250 MB of headers on the ISO). Later maybe the CEC and LED drivers the same way.
Vapor theme in the live session. Add cachyos-vapor to packages_desktop.x86_64 and set it as the live user's look and feel in airootfs/etc/skel (kdeglobals LookAndFeelPackage, no layout file, so Plasma lays out Vapor at the live login).
Drop the Boost 1.91 workaround in steamify-prepare.sh once cachyos-calamares-next is rebuilt against the repos' Boost.