First of all, I would setup the backup copy job to run daily (copy every 1 day), so that you'll have the latest state of you primary backup offsite. Otherwise, should the primary repository crash, you'll be left 29 days behind the latest backup, in the worst case. The retention can be set to the minimal of "2", so you'll always have one full and one incremental offsite. Since you want to have only one monthly full to be kept, GFS should be disabled. If you enable monthly backup and set the value to "1" you'll end up with 1 monthly Full + backup copy chain of two restore points.
The same applies if you want to run backup copy once a month, just change "copy every 1 day" to "copy every 30 days". That way the backup copy job will update the offsite full with all changes that have happened during the last 29 days. Yes, you'll still have one incremental, however you cannot go lower than 2 retention points with backup copy job.