As said, it very difficult to auto generate an 100% accurate XPath expression, since the UI layout may not always be structured the same, e.g. an UI element may appear before it.
There should be less need for the Developer Assistant app since there's an Inspect layout button to view the entire layout XML, but it's of course not as easy to "interpret".
Creating an XPath similar to the old way could be as simple as .//android.widget.Whatever[@android:text='whatever' and @android:id='@package:id/whatever'], but it would also be as inaccurate.
Do WhatsApp redraw the entire screen, e.g. move to another activity, when the send button is clicked?
That is a problem, that the app proceeds to change the layout, as a response to the click event, before Automate can generate an XPath for it.
I'll investigate if there's anyway to "select" an UI element without it generating a click event.