I have thought about it too, but the MQTT topic will receive an update about every 20-30 sek, And if I am not lucky I will not be able to set the value (of the input select) all the time, as it will be overwritten by the automation. As the data can be set by either the input select and the MQTT read-topic…
The MQTT have a read topic: my/topic/data
And a write-topic I publish to: my/topic/data/set
Then when the device have read the /set-topic and used the value, it will be published to the read-topic. Maybe the problem is the design of this topic (but I cannot change it).
This scenario (simplified, data can be A, B or C published from device to MQTT topic every 30 sec):
Automation triggers by MQTT topic my/topic/data - value: A - Sets the input select to A.
I manually adjust the input select in HA to value B.
An automation triggers on the input select change and publishes value B to the topic my/topic/data/set
Before my/topic/data/set have been read by the device it publishes the old value A to my/topic/data, which triggers the automation to set the input select back to A again. As I understand it the condition you mention cannot solve this.
Depending on timing it will work most of the time, but not if I change the input select just before the 30s update.
Maybe I just need to have another helper just to keep track of the last published value coming FROM my/topic/data, and have it as a condition.