Lessen the acidity of fruit shrub syrup
You could simply change the ratio by adding more fruit juice; otherwise you're into experimental territory by adding alkali almost at random.

Baking soda would reduce the pH, but I'm not hopeful for what it would do to the taste, or look. 

Whatever you decide, I'd test it in small portions first.
You may end up just liking it mixed with tonic water... or gin... or...