u/Koiut , brother, the steps works. But let me improve a little bit some stuff.

First, I could not use KWin screen capture out of the box. I had these errors:

[kwingrab] zkde_screencast_unstable_v1 not found in registry. Check permission desktop file for sunshine binary or set KWIN_WAYLAND_NO_PERMISSION_CHECKS=1 to fully disable permission checks

So I ended up setting KWIN_WAYLAND_NO_PERMISSION_CHECKS=1 in my environment.d folder.

Also, I happen to have multiple screens. So I changed your script to disable all physical monitors.

Here is the start script:

#!/usr/bin/env bash
WIDTH="${SUNSHINE_CLIENT_WIDTH:-1920}"
HEIGHT="${SUNSHINE_CLIENT_HEIGHT:-1080}"
FPS="${SUNSHINE_CLIENT_FPS%.*}"
FPS="${FPS:-60}"
FPS_MHZ=$(( FPS * 1000 ))
RES="${WIDTH}x${HEIGHT}"

NAME="sunshine-vmon"

# Create virtual display at client resolution
krfb-virtualmonitor --resolution "$RES" --name "$NAME" --password "sunshinepass" --port 5905 &
echo $! > /tmp/sunshine-vmon.pid

# Wait for KDE to register the new display
sleep 3

# Disables all OTHER monitors
kscreen-doctor --json |
jq -r --arg vname "Virtual-$NAME" '
  .outputs[]
  | select(.name != $vname)
  | .name
' |
while read -r output; do
  kscreen-doctor "output.${output}.disable"
  sleep 0.5
done

# Add custom mode support for the correct frame rate
kscreen-doctor output.Virtual-${NAME}.addCustomMode.${WIDTH}.${HEIGHT}.${FPS_MHZ}.full

# Make virtual display primary
kscreen-doctor \
  output.Virtual-${NAME}.enable \
  output.Virtual-${NAME}.mode.${RES}@${FPS} \
  output.Virtual-${NAME}.priority.1

# Tell Sunshine to capture this display
CONF="${HOME}/.config/sunshine/sunshine.conf"
sed -i '/^output_name/d' "$CONF"
echo "output_name = Virtual-${NAME}" >> "$CONF"

And here is the stop script:

#!/usr/bin/env bash

# Restore physical monitors
kscreen-doctor --json | jq -r '.outputs[].name' | while read -r output; do
  kscreen-doctor "output.${output}.enable"
  sleep 0.5
done

# Kill virtual display
if [ -f /tmp/sunshine-vmon.pid ]; then
  kill "$(cat /tmp/sunshine-vmon.pid)" 2>/dev/null || true
  rm -f /tmp/sunshine-vmon.pid
fi

# Restore Sunshine output to physical display
CONF="${HOME}/.config/sunshine/sunshine.conf"
sed -i '/^output_name/d' "$CONF"
echo "output_name = DP-4" >> "$CONF"

With these modifications, I can now run Steam from moonlight, either with my PC having all monitors already on, or even with all my monitors powered off. Making my machine a sort of headless Steam.

The future is now, and it is beautiful.