Any cake can appear fancy with a swirl of buttercream. I suggest you use a recipe you already make and like. It would no doubt be very good. For the dessert theater, bake a sheet cake if you feel comfortable making something that large, or several 10 inch cakes. Does the church kitchen have baking pans for your use?
I worked in every size production from retail bakeries to factory bakeries with massive equipment. Scaling up for retail bakeries is simply multiplying recipe ingredients. But when factory equipment is used, then the balance changes. Don't want to fight with anyone. This is my experience.
Event planners are usually people who can't make it doing anything else. I've been in the industry many years and I've seen them come and go. This new nitwit will knock herself out of the ring. (Her next career will be as an assistant store manager at the mall.)