It is not possible to control the suffix for instances belonging to instances group. As you can read in the documentation even making use of the SDK with the optional flags you can change the base-name
of the instance but no hints regarding the suffix.
--base-instance-name=BASE_INSTANCE_NAME
The base name to use for the Compute Engine
instances that will be created with the managed instance group.
If not provided base instance name will be the prefix of instance group name.
Moreover it is not possible to rename Compute Engine instances once they are already been created.
Here you can find an UserVoice Topic Regarding this feature.