Fin flutter isn't a factor of the strength of the bonding to the airframe, it's a function of the shear modulus of the material used for the fins, the speed of the airflow over the fin and by the fin size and shape. Especially important is the length of the tip chord and thickness of the material.
No need to guess. You can use this spreadsheet (found elsewhere on TRF) which provides a very conservative estimate of fin flutter speeds (based on a Peak of Flight newsletter - I added the shear modulus tab from internet searches). If you have AeroFinSim, it provides another data point, which I have found to generate a higher fin flutter speed.
Last point, if you use internal fillets, then the external fillets are really for improving aerodynamics and not need for bonding to the airframe. For that, JB Weld is overkill in terms of weight. Rocketpoxy is my go-to for external fillets.