The following is why we don't do cakes outside in the summer and why cream cheese is forbidden as well as whipped cream icing ......
Not to scare you.... BUT..... it's funny but true!!
OK - scary, isn't it? Your cake is made with butter and or cream. Even the cheap fake stuff from your local grocery store bakery is made with shortening - it all melts the same. This is also true of indoor spaces. I once asked if the room the cake would be in was air-conditioned. I was told, "yes, it is!" Well, folks, I get there and yes, technically, the room was air conditioned. The people had it set at 78 degrees. It is humid and it's Mississippi. 78 degrees, inside, in Mississippi, is not air conditioned as far as the cake is concerned. It must be set to no more than 70 degrees or it has to be refrigerated until 10 minutes before serving. I stood there and watched the buttercream flowers melt off the beautiful cake - in minutes! It was so sad. The topper fell off the cake as the buttercream softened. There was nothing I could do about it.
There are no refunds when your cake falls apart because it is too hot. I understand A/C is expensive. But on your special day - crank it up. There is no point in spending all that money on such a beautiful cake just to watch it melt before your eyes.
Been there - Done that!