There is a gluten free dairy free bakery at Disney Springs called Erin McKenna’s Bakery. I have never been there, but have heard good things.
It is good, but $$ for what you get. I have not tried to order a big cake, but my extended family has gone there for treats.
Second, we have not ordered anything custom from the GF or Contemporary's bakery, but I have seen some of their creations and they can be AMAZING! A few times when staying at CR or dining at Cali Grill we happened to see cakes being delivered. When dining at Cali grill, you are allowed to go out on the balcony, and on the way out you walk past the banquet rooms.
The flowers they can do are also AMAZING. I am sure they are very expensive, but they make some gorgeous displays!
On top of that, WDW chefs are very talented at working in chocolate.
I am not sure where he is currently working, but WDW employs a chef who goes by the nickname "Chef TJ." He has worked in a number of locations over the years. I'm not sure how to say here, but he has long been employed as a 'special needs' chef. If someone with an allergy dines at a TS, and requests to talk to the chef, that is the person who (generally) talks to them. Chef TJ's real talent is vegan cooking, but he also takes care of folks with food allergies. If you happen to find where he is working, he is a super nice chef.