RCD snubber is the appropriate method to absorb primary overvoltage caused by transformer leakage inductance. You have to check the actual primary voltage and current waveforms, an additional RC snubber will unlikely improve anything.
The transformer air gap should be sufficient to avoid transformer saturation. If Bmax is actually 260 mT, transformer core will run very hot, but not cause problems for the transistor.
Are you sure that the converter is operating regularly with constant duty cycle? Hard to determine without oscilloscope measurement.