OK, I have tried the MQTT discovery option.
And it looks like it would be possible to generate a sensor or binary_sensor for each device_tracker entity and set the STATE to Online / Offline or whatever you prefer.
But then I found that I still want to customize my friendly name and the icon for online and offline and maybe a few other things. Overall it won’t really make the manual process go away …
It was a nice exercise and I learned some things about the MQTT and discovery, but I will go back to the manual process.