What is the switching frequency, 150-650kHz fixed or spread spectrum?
What are the switching oscillations on the output, noise, ripple? It may be necessary for an output filter instead.
Are they excessive in boost mode, in buck mode, or both?
Trial and testing will be necessary, solder components to the PCB, you may have to be creative and scrape off solder mask, etc.
Putting the snubber across M2 or M1 should not make much difference. You can test it for yourself. The DC voltage across the snubber may differ though.