Flutter is one of those aerodynamic phenomena that is partially a black art. Stiffer controls and surfaces do help. It is still a fact that wide control surfaces flutter more easily than narrower control surfaces. Also, as you have noted, flutter occurs at higher speeds. Most 3D planes can not fly very fast or they will flutter. This is just a fact of life with a 3D plane.

Another thing that has not been mentioned is the trailing edge of all control surfaces need to have sharp corners. For the wide controls, leave, or make, the trailing edge square. A straight line taper from the leading edge of the control surface to a sharp trailing edge is best. Pylon racers generally insert some 1/64 ply in the TE so they can have some strength when they taper to a sharp edge. Next best is to leave the TE corners square and sharp. I know it looks nice to round them off, but rounded corners are the worst for flutter.

It is my opinion that many manufacturers do things to planes to promote sales and if they kept the square TE, potential buyers might think they were cheap for not taking the time to round off the TEs and maybe not buy the plane. If the plane with the rounded TE does get flutter, their answer will be, "You were flying too fast."

The same thing happens with Sticks and other high wing sport planes. These planes need anhedral to counter roll due to rudder just as low wing sport planes need dihedral or sweep. The manufacturers are afraid, in my opinion, to use anhedral for fear that people will think the plane is unstable and not buy it. Just take any Stick up, even the flat wing Ultra Stick, and put in rudder. It rolls like a trainer. It has nothing to do with stability unless the plane is a trainer that you want to recover by itself. They will tell you how to use mixing to fix the problem.

The photo shows the aileron on an old Morris Hobbies TopCap profile. Their planes were popular when 3D first started and they were the first to hover and harrier. Their planes were designed and extensively tested in the US.