If you are set on taking the MCAT in August, get the rest of your primary and secondaries completed as soon as possible with the pending MCAT, as long as you are confident that you will do well on it.
Remember that once you send in your primary, it takes 3-4 weeks for AMCAS to verify it, so if you wait until you get your MCAT score to submit, then you'll have to wait another month to start filling out secondaries, sometime in October.