The wiring is wrong. The main triac should be in series with the heating element, not across it.
Because the thermal inertia of the heating element in the oven prevents it rapidly heating or cooling, it would be better to simply use the triac as an on/off switch operated by the temperature sensor. Using a zero-crossing opto-isolator to drive the main triac is sensible and safe. Use a zero-crossing type rather than a normal type to reduce electrical interference but I would forget trying to use phase control for this application.
Brian.