It maybe mature technology, but unfortunately a lot of knowledge has been lost along the way.
The airfoil section is not all that critical, but designs with semi-symmetrical airfoils fly much better than flat bottom airfoils. What makes a design fly out of dives when trimmed is the decalage (relationship of the wing incidence to the tail, engine thrust line, and balance. The airfoil selection has little to no effect, because I can setup a fully symmetrical wing to do the same thing. It is primarily having a nose heavy design that makes the airplane recover from a dive with the extra incidence. In addition, you need a pretty generous horizontal tail to achieve a slow landing when nose heavy so the you can flair.
I personally hate lite ply fuselages, they are heavy so a lot of wood is removed to compensate, but a well designed and built balsa design with careful application of aircraft plywood in key locations will withstand far greater abuse at the same weight.
I agree that a shoulder or high wing design is preferred with modest dihedral and trike gear. But having a provision for switching to a tail dragged would be a plus. One other thing is a constant chord wing only.
My main complaint with most trainers is that they mount the tank too low for proper fuel draw. I also think that beginners should fly nitro powered models because you can just refuel again and go.
Rubber bands are fine, but you have to put them on correctly or they may not come off readily. Having them crossed like Jaka shows in his photo is fine once you are not banging the winging every landing, but straight forward and aft bands allow lower forces to release the wing. Of course if you really want to do the wing right, the servo should not extend below the point where the wing saddle meet the wing. In the old days, that meant laying sideways with pushrods out to bellcranks, but today outboard servos work fine for little cost. However the reason that bolt on wings cause damage is because everyone uses at least a pair of 1/4-20 nylon bolts and that is way too strong to shear and leave the wing undamaged. 10-24 bolts are plenty strong until you get far heavier and bigger than a 40 sized trainer.
About the closest anyone comes now is the Sig LT-40. 40 years ago the Sr. Falcon flew very well. Base any new design on either of these designs and you can't go too wrong.