Updating since constructing this post has led me to at least a partial understanding!
I have this switch included in Z2MQTT:
I’ve put the bit in configuration.yaml and see the output of the top button in the Z2MQTT log:
z2m:mqtt: MQTT publish: topic 'zigbee2mqtt/4-button switch A', payload '{"action_brightness":254,"action_transition":0,"battery":43,"brightness":254,"color_temp":370,"linkquality":152,"transition":0}'
and
z2m:mqtt: MQTT publish: topic 'zigbee2mqtt/4-button switch A/action', payload 'on'
Pressing it again yields a complex first message with the second message:
z2m:mqtt: MQTT publish: topic 'zigbee2mqtt/4-button switch A/action', payload 'off'
As my trigger, I used " When an MQTT message has been received" with topic “zigbee2mqtt/4-button switch A/action” and payload “on” (no quotes in the UI entries), it works, using toggling the Ecosmart bulb switch as the action (although I need to press it twice, understandably)!
And “brightness_up” and “colortemp_up” work for the 2nd and 3rd buttons from the top as payloads (without quotes),