A good rain on your first outing with this tarp, excellent! What a way to build confidence in what you've made, and not better way to enjoy it!
As for cooling things down a bit... loosening the UQ to allow a small air flow can help. Hard to understand that you were too warm at 50F and rain with a 40F bag. Guess you are as warm a sleeper as I am! What most rate to 40F I take down below 30F. My answer is to use a lighter weight bag, or poncho liner - my preferred TQ in temps above 50F.
BTW - this sounds like an awesome trip! Thanks for sharing!!