My instinctive answer is yes, you'll have trouble...if you want a decent seat for Fantasmic. The average in-park TS meal takes about 60-75 minutes, depending on the number of courses, how busy the restaurant is, etc. Assuming you're seated on time, you'll finish up around 6:30 pm. That gives you about 30 minutes to get to Fantasmic and find a seat. It's not a short walk to the theater, so I'd alot at least 10 minutes for that part alone. That means you'll be walking into the theater about 20 minutes before the show. They recommend folks get there at least 45 minutes early to get a good seat. Given that you're going during a marathon weekend, I'd guess the crowds will be high, so the showing will be popular. It probably also means that the restaurants will be busy, so there is a real chance you'll be seated late.
Overall, I think you can make it, but it'll be tight and you probably won't get a very good seat at Fantasmic. Personally, I'd aim for an ADR about 2-2.5 hours before the show if you didn't want to have to stress about seeing Fantasmic.