Hello again. Some further testing proved that my initial setup was inadequate, as it did not address the scenario where I was already home (and in Home mode) at Sunset, in which case ST does…nothing. To get around this, I still had to manually get into Away mode using the Goodbye! routine, then immediately go back into Home mode using the I’m Back! routine, at which point the additional lights would come on. Not ideal.
I ended up using @csuk’s instructions above to create the additional modes “Home PM” and “Away PM” (note that these now have to be added using the ST IDE web interface, as the mobile app recently dropped the ability to add new modes. See explicit instructions for how to do this from @JDRoberts’ 9/16 post in this thread.) and the additional routines Home to Home PM, Away to Away PM, and Away PM to Away.
This made my brain hurt a little, but essentially, ST now uses routines to change modes at Sunset, then use presence based on cell phones within the Smart Lighting app as the trigger to turn the additional lights on only when the mode is Home PM. The logic behind this is a bit complicated and I need to make sure that this behaves as expected under all scenarios (such as a 4 am arrival time on a weeknight), but so far so good for the normal routine of arriving home from work after dark and the extra lights come on. I will update again as I nail down the rest of the pieces.