There Are quite a few community members who also use home assistant or have experimented with it.
But to be honest, for the purposes of this thread, which is in the iOS section of the forum, by far the easiest thing is just to get a $10 Meross WiFi smart plug which will be visible to both Apple HomeKit and smartthings. That’s all you need as long as HomeKit presence is working, and generally historically it has been the most reliable since it is built into the iOS operating system. You just have Apple presence turn it on and its turning on will be visible to smartthings. Essentially the iOS equivalent of Bixby routines. 
How to Use a Meross Plug to Use HomeKit Presence to Trigger ST Routines
There are at least a dozen different topics in the forum on presence, I don’t think we have to try to shove everything into this one. Particularly since there is such a simple solution for those with iOS devices.
You can find the other topics on presence on the following quick browse list:
Topics tagged project_presence
Submitted with respect.