I agree with changing the gear. I wonder if it would benefit from clipping the wings 1 rib like the 4-Stars. You might also look at enlarging the rudder and maybe the elevator.

I have a Sting Ray 40, borrowed from a buddy to test engines. It is the Mid-Star clone, whereas the Thunderbug is a 4-Star clone. The gear style is reversed from their Sig counterparts. It, like the 4-Star, flies much better with the wings clipped.

I didn't build the plane, but I like the Balsa USA ones because they have sheeted leading edges. The Sig planes, with multi-stringer leading edges, have a tendency to pick up warps and are less twist resistant than planes with a sheeted LE.