First, when you have only 1 kid especially daughter, in future when you grow old and she married, and something bad happen to either you or your wife. She have her own family matter to attend and still need to spare her time to her parent problem too. This will burden her so much....

If something big happen, at least your daughter have sibbling to discuss with, or even financial sharing or time to support their parent in future.

Adding another kid wouldn't take away more of your "ME-Time". Somemore your daughter wont have a big age gap to your 2nd child. All the activities can be done together once at a time.

I believe your wife will done a great job taking care of them.