Hello: Just saw this post now.
Snubber power is a very old and well known question. It should be perhaps a few % of the max output power, so 5W is a lot for a small supply.
The ringing is a natural result of energy stored in the leakage inductance of the magnetics after switch turn off.
Minimum load is a worst case, as network damping is reduced.
The PRF does not affect the ring freq, only determined by the Llkg, and parasitic inductances of the magnetics, combined with the snubber R-C.
Many flyback and forward converters use active snubbers, diodes, TAZ or extra windings to reduce the lost power of the required snubber.
Old Vicor patents are an excellent example.
To learn more the OP could search sites about converter forward and flyback design, and snubbers.
Enjoy,
Jon