If you really want to see all three sites, you can probably do according to your plan.
Yet, both the Sagrada Familia and the Parc Guell are places where you could easily spend much more time depending on your interest.
At both sites you will benefit from a guided tour or an audio guide to see and understand much more.
If you are really interested in the basilica, do take a guided tour first and then continue on your own, assisted but nor limited by the audio guide. Include the towers.
Regarding queues at the port: Probably most people will not take a taxi but the shuttle bus to the base of the Ramblas. You can get a taxi there, too, and have access to the metro, but with your schedule a taxi directly from the port is the best option (and should not be a problem to get),