Hello Boris, I am not involved in packaging gdm or orca but tried to get more information. I added also the a11y tag in the hope to get the attention of the right people.
I tested inside a minimal VM with the gnome package installed. In the gdm3 login screen I also got no reaction from the Ctrl-Alt-s shortcut, neither in Wayland or Xorg mode. Your second command did not work for me, I adjusted it and prepended a shell before the eval: $ sudo -u Debian-gdm bash -c 'eval $(dbus-launch) ; export DBUS_SESSION_BUS_ADDRESS DBUS_SESSION_BUS_PID ; GSETTINGS_BACKEND=dconf gsettings set org.gnome.desktop.a11y.applications screen-reader-enabled true' This seems to modify the setting but does not immediately activate orca - just after a reboot speech output worked. I guess this happens because of using another dbus process. When using the already running dbus it seemed to work immediately: $ sudo -u Debian-gdm bash -c 'export DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/$(id -u "Debian-gdm")/bus; GSETTINGS_BACKEND=dconf gsettings set org.gnome.desktop.a11y.applications screen-reader-enabled true' The Debian Wiki has also a shell before the eval command. And uses instead of sudo the su command, which from a regular user requires the not existing password for Debian-gdm. The Wiki does also not mention the requirement of a reboot. https://wiki.debian.org/accessibility#gdm_accessibility I guess if this "activation" command could be hidden in a short-named helper script would be helpful. Kind regards, Bernhard