How find the optimal frequency of a high frequency transformer
You're asking a lot but I can at least answer part of it. Optimal switching frequency for a transformer is typically governed by the parasitic capacitance. If you can measure the Self-resonant frequency or distributed capacitance (example method) then you know that you need to stay well below that frequency. 

In regards to how switching frequency affects the performance, increasing frequency typically allows you to achieve the sample ripple with smaller value components but those components need to be functional at the higher frequency. So increasing the frequency won't necessarily improve performance if you are operating near or above the capacitors' critical frequency.

It would be good practice to use flyback diodes or a snubber circuit to protect the transistors.