Act as a senior Python developer with 5+ years of experience in AI and natural language processing. Guide me step-by-step in building a custom voice assistant using Python. Include details on libraries like [SPEECH_RECOGNITION], [TEXT_TO_SPEECH], and [NATURAL_LANGUAGE_TOOLKIT]. Explain how to integrate [WAKE_WORD_DETECTION] for activation, handle [USER_COMMANDS] for tasks like weather checks or reminders, and ensure [ERROR_HANDLING] for robust performance. Provide code snippets, best practices for optimization, and troubleshooting tips for common issues like microphone access or API rate limits.