I think there is an issue with the SmartThings Android app. You are not the first one to mention that issue. I know there are other apps that don’t configure properly. I can take a look and see if I can remove some the the help text to see if that clears up the problem, works fine on iOS.
I don’t know if that dimmer works. I use this dimmer along with my DH and SmartApp manager for the dimmer to control the groups. Both are covered in this post. The dimmer I posted is battery operated and looks just like a regular dimmer.