There are diabetic friendly cheesecakes at Sci-Fi and 50's prime time that were really good. I believe Grand Floridian has brought back the Pana Cotta, which is sugar free, so Olivia's may have as well and it is really good too.
Erron McKenna in Disney Springs has some options too.
All of the Starbucks locations can make sugar free frapracinnos (flavors may vary from location to.location).
Almost all of the ice cream places have at least one or two sugar free flavors.
And I don't remember where, but there was somewhere that did a non-alcoholic mixed drink that was sugar free.
In some of the candy shop, you can find boxed dark chocolate covered pretzels that are sugar free.
Before the pandemic, there were getting to be A LOT of options, unfortunately a lot are gone since the park reopened. One of the people who really was pushing them to do this started her own business during the closures to make ends meet and last I had heard didn't return to Disney, but you can find her and several others that will make sugar free treats (anything from cookies to cakes, pies, even cheesecakes) on a Facebook group called Ears For You that are cast members that were/are affected by the closures and they will deliver to Dis ey and other nearby resorts.
Also, a word of caution, almost everywhere at Disney World is switching from light and zero sugar lemonade to regular lemonade, so be sure to ask and confirm what they are using if you get lemmonade.