If you have time, I would try to shadow your plans for a few days to see how things work on the My Disney Experience app. If you're going on a Friday, try to see what times look like after the park opens through the first hour tomorrow morning. I would think Space Mountain would make the most sense to do first but it all depends on what you want to do the most? Splash Mountain, I've seen lately, isn't always open at 9am EST. It can sometimes be a bit delayed. It might make sense to do Space Mountain and then head over to Splash/BTM after that and save Buzz for later.