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

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