I think you are overthinking this. You don't need to change motors. To use a 3 blade prop, just go down by 1 inch and keep the same pitch and brand. A watt meter is a good tool to have when you want to experiment as it will tell you exactly how much power you are using instead of guessing and hoping for the best.

As for you takeoffs troubles, small planes don't deal well with grass. That's a weakness you just have to accept when you choose to fly an electric park flyer instead of a traditional .40-.60 size trainer.