Interesting discussion. First off, while I think we nailed the reason for the ballooning, I think it is bad advice to recommend to a relative newcomer to start adjusting the rigging (incidence, thrust, decalage etc.) of a perfectly proven design. Especially without the ability to measure these things to know exactly what is going on. i.e. having an incidence meter.

I have found it best to have a lower pitch prop on a trainer as it gives more efficient thrust. I own this engine, and an 11-5 would be a great choice. There is no reason to have speed in your trainer. That creates a lot of issues. The controls will get more sensitive, you have to turn around faster, to keep from getting to far away, and overall, they will not have as much time needed as they would otherwise to make correct decisions. Also the lower pitch gives good breaking effect to slow down your landing approach.

While we are drifting somewhat off topic, let me take us further. I am an instructor in my club. One of the things I would like to emphasize to all of the instructors out there, is to emphise flying right hand AND left hand patterns and landing approaches. I cannot believe how many pilots in my club that cannot land to the right. This is a bad practice to maintain, and it takes practice to get over it. It is really not safe to not be able to fly both ways, and I feel you have not completed your training until you have the ability to land both ways.