If you were not ascending a tower I would recommend the first time-slot at La Sagrada Familia, because there would be no hold-over people in the church when you walked in. But with a tower ascent, I think you need to go line up for the tower right away, so the church will have lots of visitors when you're ready to see the lower level, no matter what time you go.
I spent about 90 minutes at La Sagrada Familia without a tower. I used the audio guide and liked it. I do find that I spend more time in churches and museums when I use an audio guide. I went to the small museum at LSF, which focuses on design/construction. I found it quite interesting and it was blessedly free of other visitors. I also spent 5 minutes or so in the shop. Be aware that once you enter the shop you cannot return to the church, so be careful about that.
I don't think LSF, Sant Pau and Parc Guell are too much for one day, but you'll be on your feet a lot. Sant Pau is a multi-building complex with nice grounds. I spent 2 hours there and had to hurry to finish before it closed. I believe an additional building has opened since my visit, so I recommend allowing longer. Meal hours in Barcelona are late, so if you book an early LSF visit you can go to Sant Pau before lunch and you won't need to worry too much about how long you spend there. Long visit? Grab a quick lunch in the area. Shorter visit? Have a leisurely, sit-down lunch. I don't think I'd book Parc Guell for before 4 or 4:30 PM, but I haven't checked the closing time. If you get there a lot earlier than your ticket time, you can go first to the free-entry area.