I don’t do tts much any more. The family thought it was creepy. But I do know that what I used to do and what people used to recommend was to add in delays between command to tts to give it time to finish. A quick count of words can be used as a reasonable factor.
Centralize all the commands to tts in one role using the Command Item approach from Design Pattern: Separation of Behaviors and then use Design Pattern: Gate Keeper to queue up and space out the calls to tts. You can add a limit there too to count the number of characters sent for the month and generate an error when you get over 5000.