Light switch recommendation for virtual 3-way with battery-powered auxiliary?

Sure, you can actually use any battery powered device that can talk directly to the smartthings hub and create a virtual three-way. There are a number of options in the buttons FAQ, which I will link to below.

All of that said, I think your best option at this point is to use a Lutron Caseta master switch and the Lutron pico. These are well engineered, work well together, and there is now an official SmartThings integration. You will need to buy the Lutron smart bridge to get the integration, but if you buy it in the starter kit, it only costs about $20 above the switch costs. ( One bridge can handle up to 40 Lutron devices.)

And here’s the button FAQ if you want to look at the other options that work with SmartThings. (This is a clickable link)

Some of the devices in the FAQ are battery powered, some are mains powered, some are handheld, and some are wall mount, so read the descriptions carefully: