Calling client based on a timer

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I have created a node to read serial data from an Arduino in the form of bytes. I had to do this because the Arduino does not support micro-ros or rosserial. I had to run the callback that reads this data on a timer using self.create_timer. Every time data is properly received, I am using a service to decode this data and return it. My problem is that when I create a service and call it every time data is received the service completes its entire task, but nothing is ever returned.

Here is a stripped down version of my node.

class SerialReader(Node): 
    def __init__(self):
        super().__init__('serial_read_node') 

        # Declare parameters
        # Initialize serial connection

        self.cli = self.create_client(
            DecodeMessage,
            'decode_serial_message'
        )
        while not self.cli.wait_for_service(timeout_sec=1.0):
            self.get_logger().info('service not available, waiting again...')

        self.req = DecodeMessage.Request()
        self.create_timer(1, self.receiveData)

    def receiveData(self):
        # Receive bytes from Arduino as 'message'
        result = self.sendRequest(reading_bytes)

    def sendRequest(self, message): #call client for data
        self.req.message_received = message
        self.future = self.cli.call(self.req)
        rclpy.spin_until_future_complete(self, self.future)
        self.get_logger().info(f'Future = {self.future}')
        return self.future.result()

def main():
    rclpy.init()
    node = SerialReader()
    rclpy.spin(node)
    node.destroy_node()
    rclpy.shutdown()

if __name__ == "__main__":
    main()

My question are:

1. Is my problem that the client is not its own node?
2. Is there a way to use rclpy.spin_until_future_complete() on a callback function?
3. Is multithreading and using executors a path to solving my problem?

I did not include my service as I have found that I can print to the logger the line before the return statement with everything working properly, but nothing here gets past the line rclpy.spin_until_future_complete(self, self.future).