I'm finding with the Ah's especially (and Yaws) that even though I've taken a deep, low breath, I'm using too much air on the singing and need to take another in the middle. Advice?

For the time being, feel free to add a consonant (so instead of singing AH or YAH, you might sing MAH, NAH or BAH). The addition of a consonant should help you with airflow control!