I use two, but if you already use HomeKit and your iPhone presence is reliable, the easiest thing is just to use an inexpensive Meross device (which can be seen simultaneously by both HomeKit and smartthings) as a proxy. There’s a how to post here:
How to Use a Meross or Matter Plug to Use HomeKit Presence to Trigger ST Routines
My older solution involves using an ibeacon and the Geofency paid app. This has worked very well for me and doesn’t require setting up HomeKit. You can read about this and other possible solutions in the following thread. It was in response to a UK user, but all the same solutions will also work in the US:
iBeacons+iOS+SmartThings (UK) presence [updated in 2023] - #2 by JDRoberts