I wrote about excess EMI. And in this usage their role is exactly the limiting of the 'sharpness' of the switching, since that is usually one significant source of high frequency noise.Snubbers are used to protect semiconductor switch components, they are NOT used to reduce switch fundamental output noise.
- I would first try to find others who built this circuit and ask for reference. Right now nobody here (you included) has any idea if the measured spike is 'normal' for this circuit or not.
- I would also try to make a guess about the energy contained in that spike. I could not find the exact output voltage/current of your DCDC here, so I have nothing to compare that 0.8V to. At what current and voltage it is 0.8V? What's the value when it is running at full power? All that filtering might be just useless against a spike which has significant energy in it.
- I would try to google up some design guides for this kind of 'float' based inverters, where the coil actually has a cap in series (!!!) to a middle of a cap bank. I have a feeling that the frequency and the size of those caps has something to do with the inductivity of the transformer coils: and that is exactly one parameter you have just modified in your build with picking a bigger core.