If it operates like the other doorbell cams, you can install a relay directly on the power lines to read the voltage drop when the doorbell button is pressed. The relay can then be used to trigger a home automation device.
This is a good alternative if the doorbell isn't sending button presses through ONVIF events
I'm not sure how there could be much of a voltage drop just from pressing a button which does not actually cause a physically connected chime to ring. It does send a 433MHz signal of course but again I don't think that would cause much of a voltage drop either, if any.
Do those other doorbell cams also only trigger a wireless chime or do they support a connection to a physically connected standard doorbell chime? With a physically connected chime what you say makes sense.
Or maybe I'm missing something or misunderstanding what you're saying.