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Replace rclpy.shutdown() with self.destroy_node(). Please see the code snippet:

# Copyright 2016 Open Source Robotics Foundation, Inc.
# license removed for brevity

import rclpy
from rclpy.node import Node
from std_msgs.msg import String


class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1
        if self.i==10:                   # I could put other non hard-coded tests here
            msg.data = 'Done'
            self.publisher_.publish(msg)
            self.get_logger().info('Publishing: "%s"' % msg.data)
            self.destroy_node()           # <-- Here is where I want to the node to quit

def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Below is the info it prints at terminal:

$ ros2 run examples_rclpy_minimal_publisher publisher_member_function 
[INFO] [1663241782.469494158] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [1663241782.963022122] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [1663241783.464649394] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [1663241783.963231469] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [1663241784.464505740] [minimal_publisher]: Publishing: "Hello World: 4"
[INFO] [1663241784.964727795] [minimal_publisher]: Publishing: "Hello World: 5"
[INFO] [1663241785.464552693] [minimal_publisher]: Publishing: "Hello World: 6"
[INFO] [1663241785.963467024] [minimal_publisher]: Publishing: "Hello World: 7"
[INFO] [1663241786.464590671] [minimal_publisher]: Publishing: "Hello World: 8"
[INFO] [1663241786.965043845] [minimal_publisher]: Publishing: "Hello World: 9"
[INFO] [1663241786.966554516] [minimal_publisher]: Publishing: "Done"

Replace Please replace rclpy.shutdown() with self.destroy_node(). Please However, I am not sure if destroying a node inside a callback is a good idea. Nevertheless, please see the code snippet:

# Copyright 2016 Open Source Robotics Foundation, Inc.
# license removed for brevity

import rclpy
from rclpy.node import Node
from std_msgs.msg import String


class MinimalPublisher(Node):

    def __init__(self):
        super().__init__('minimal_publisher')
        self.publisher_ = self.create_publisher(String, 'topic', 10)
        timer_period = 0.5  # seconds
        self.timer = self.create_timer(timer_period, self.timer_callback)
        self.i = 0

    def timer_callback(self):
        msg = String()
        msg.data = 'Hello World: %d' % self.i
        self.publisher_.publish(msg)
        self.get_logger().info('Publishing: "%s"' % msg.data)
        self.i += 1
        if self.i==10:                   # I could put other non hard-coded tests here
            msg.data = 'Done'
            self.publisher_.publish(msg)
            self.get_logger().info('Publishing: "%s"' % msg.data)
            self.destroy_node()           # <-- Here is where I want to the node to quit

def main(args=None):
    rclpy.init(args=args)

    minimal_publisher = MinimalPublisher()

    rclpy.spin(minimal_publisher)
    rclpy.shutdown()


if __name__ == '__main__':
    main()

Below is the info it prints at terminal:

$ ros2 run examples_rclpy_minimal_publisher publisher_member_function 
[INFO] [1663241782.469494158] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [1663241782.963022122] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [1663241783.464649394] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [1663241783.963231469] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [1663241784.464505740] [minimal_publisher]: Publishing: "Hello World: 4"
[INFO] [1663241784.964727795] [minimal_publisher]: Publishing: "Hello World: 5"
[INFO] [1663241785.464552693] [minimal_publisher]: Publishing: "Hello World: 6"
[INFO] [1663241785.963467024] [minimal_publisher]: Publishing: "Hello World: 7"
[INFO] [1663241786.464590671] [minimal_publisher]: Publishing: "Hello World: 8"
[INFO] [1663241786.965043845] [minimal_publisher]: Publishing: "Hello World: 9"
[INFO] [1663241786.966554516] [minimal_publisher]: Publishing: "Done"