The gap filler voltage is soft and disappears when loading the inverter. As you can see, loading with a 40W bulb still leaves a notable part of this voltage, it disappears largely at a full power ...

The output transformer has a 9V-0-9V, 2 Amps on the secondary and 230V on the primary. Use suitable heat-sinks for MOSFETs. Remember that this is not a sine-wave inverter, not even a modified one.