If you just like to build, and have the budget, by all means go ahead and get a 4* or Super Sport, or any of the dozens of intermediate models that are available. You can't go wrong with any of them.

That being said, with nine solo flights you are a long way from exhausting the possiblilties of your trainer. The Happy fly 20 has no dihedral, and is more of a shoulder wing than high wing. The proportions are more like a "stick" than a typical trainer. If you move the cg aft a little and increased the throws a little you would have a very capable aerobatic machine. With the cg at the aft limit, it would even be a little wild. You may begin to master that airplane after a hundred flights. Get an experienced model pilot to give you a demonstration with your Happy Fly and I think you will be amazed.

To address your general questions:

Airfoil - The only advantage of a symetrical airfoil is that it flys upside down with the same angle of attack as right side up. It is not quite as efficient as a cambered airfoil but is pretty well standard on aerobatic airplanes. The difference in flying characteristics between symetrical and cambered is not much. The "ballooning" tendency mentioned by MinnFlyer is more a function of cg position than airfoil shape. A forward cg is very stable in pitch and will pitch up sharply with speed increase, whereas a rearward cg is less stable and will not pitch up much (balloon).

Dihedral- Dihedral causes an airplane to roll away from a sideslip. If a wing drops due to a gust, or control input, the airplane will slip toward the low wing and if it has dihedral will roll back toward level. In aerobatic flight, in maneuvers that involve a sideslip, dihedral requires aileron input to prevent the roll. This is most apparent in knife edge flight where the plane is in a constant sideslip, and if it has dihedral effect it will try to roll out. This is sometimes referred to as "roll coupling".

Performance- With any airfoil, a light wing loading provides better performance and slower landing speed. Landing speed is not effected by dihedral and cg position.

Tricycle landing gear- The gear configuation only affects the way it taxies. Going to a tailwheel configuration gets rid of the drag of the nosewheel and results in a little better performance.