Hiya Mark,
Nest is not directly supported in smartthings but people on the community have given a few different methods as a work around. Not sure if you have given this a go The instructions buried in here helped me get my 2nd gen nest working. [UK] Nest Thermostat Device Type
I’m not sure 100% if this will support some of the new features of nest like the hot water controls, i’m sure someone can provide better insight,