Too tired to answer in detail today but the battery powered popp/Devolo switches work well with SmartThings, come in many different color options, and have a very good custom device type handler created by AdamV. The popp Switch talks to the hub and the hub talks to whatever micro is controlling the light. ( or to a smart bulb if you are using those.) So you can put your two-way auxiliary switch anywhere you want.
That’s probably the most popular option in the UK for a virtual two way using a battery powered device, and it’s a good one.