Switch Steam Deck desktop user with a single sudo command.

switch-session-user.sh

#!/bin/bash
user="$1"

if ! [[ `id "$user" 2> /dev/null` ]]; then
    echo "Not a valid user" 1>&2 && exit 1
fi

if [[ `echo "$user" | grep "|"` ]]; then
    echo "Username cannot have '|'" 1>&2 && exit 1
fi

if [[ `grep "User=$user" /etc/sddm.conf.d/steamos.conf` ]]; then
    echo "User already set" 1>&1 && exit
fi

sed -i "s|^User=.*|User=$user|" /etc/sddm.conf.d/steamos.conf
/sbin/systemctl restart sddm

Usage: sudo switch-session-user.sh username

Every desktop user will have an entirely different Steam install, so prepare to waste space. It makes it easy to share your deck with other people though.

If you want to force a specific user on boot, you can create a systemd service:
/etc/systemd/system/default-session-user.service

[Unit]
Description=Sets a default session user

[Service]
Type=oneshot
ExecStart=/bin/bash /path/to/switch-session-user.sh username

[Install]
WantedBy=multi-user.target

sudo systemctl enable default-session-user.service