For a 5:30 flight your assigned DME pickup will be around 2:30. I'd say 2 PM to be safe because they do pick up earlier than 3 hours out, plus your letter will tell you to be there before the scheduled pickup, plus I like a buffer. Give yourself a few extra minutes to pick up luggage from Bell Services if you'll need to do that.
Leaving Disney Springs, figure 90 minutes or so to get back to Pop (unless you plan on using rideshare). The bus from Disney Springs isn't as frequent as the park-to-resort bus, you could wait a while for a bus to pick you up there. That puts you as leaving lunch by 12:30. Depending on the restaurant, figure 60-90 minutes to eat. I wouldn't book the ADR to start after 11/ 11:30 to be safe.
Rideshare back to Pop buys you some time, but whatever you plan on, plan to be back at Pop by around 2 pm.