You use gdbus directly to send notifications

gdbus call \
        --session \
        --dest org.freedesktop.Notifications \
        --object-path /org/freedesktop/Notifications \
        --method org.freedesktop.Notifications.Notify \
        "App_name" 0 "audio-headphones" "Title" "Message" '[]' '{}' 5000 \
    | sed -E 's/.uint32 ([0-9]+).*/\1/' \
    | xargs -I{notification_id} sh -c 'sleep 1 && gdbus call \
        --session \
        --dest org.freedesktop.Notifications \
        --object-path /org/freedesktop/Notifications \
        --method org.freedesktop.Notifications.CloseNotification {notification_id}'

I also made this function that additionally allows to update the notification, and the timer for forced closing will be refreshed

notify() {
    local app_name="$1"
    local title="$2"
    local message="$3"
    local duration="$4"
    local notification_file="/tmp/${app_name}_notification_id"
    local timer_file="/tmp/${app_name}_timer_pid"

    # Read existing notification_id if available
    local notification_id=0
    [ -f "$notification_file" ] && notification_id=$(cat "$notification_file")

    # Cancel previous timer if it exists
    if [ -f "$timer_file" ]; then
        kill $(cat "$timer_file") 2>/dev/null
        rm "$timer_file"
    fi

    # Send notification and save new id
    new_id=$(gdbus call \
        --session \
        --dest org.freedesktop.Notifications \
        --object-path /org/freedesktop/Notifications \
        --method org.freedesktop.Notifications.Notify \
        "$app_name" "$notification_id" "audio-headphones" "$title" "$message" '[]' '{}' "$duration" \
        | sed -E 's/.uint32 ([0-9]+).*/\1/')

    echo "$new_id" > "$notification_file"

    # Schedule closing of notification
    (
        sleep "$duration" 
        gdbus call \
            --session \
            --dest org.freedesktop.Notifications \
            --object-path /org/freedesktop/Notifications \
            --method org.freedesktop.Notifications.CloseNotification "$new_id"
        rm "$timer_file"
    ) & 
    echo $! > "$timer_file"
}

Usage:

notify "App_name" "Title" "Message" 5000

As long as you use the same App_name the notification will be updated.