Why are you using SmartThings then? Wouldn’t it be simpler to just add a Z-wave dongle to your existing HA server and do everything in Python? Then you wouldn’t have to touch Groovy.
Your SmartApp will need to be given a list of devices that it’s able to access. There’s plenty of examples on the forum. The HomeBridge SmartApp is likely to do what you want (i.e. two-way getting and setting via a SmartThings endpoint): SmartThings + Raspberry Pi & Homebridge = Siri HomeKit Controlled Smart Home
I think I’ve also seen a standard REST API SmartApp somewhere, but I can’t seem to find it atm.