You named the real problem. 30-minute epoxy is too thick to properly soak into the balsa. This creates a "surface bond" joint that looks and feels strong until extra force is applied. Then it breaks. I have not used any type of epoxy for construction in my planes in the last 20 years. I use exclusively thin and thick CA. To date, I've had no in-flight structural failures on sport planes, pylon racers, warbirds, and giant scale planes.

Cut the joint apart, peel/scrape off all the epoxy, and reattach using thin CA. The tristock mentioned by other posters is a good reinforcement method and can also be CAed in place.

Dr.1