I do not recommend updating your devices automatically.
The general rule is : if it is not broken, do not fix it (in other words: you might brake something that works).
In the current setup, the ota_notify implementation will download the available firmware for all devices but only notify the device that is referenced in the service call.
There other ways to filter only the zha devices (zha entities).
Anyway, currently you would best do an ota_notify on one of the devices only with download set to true, and then loop over all the devices with download set to false.