You have to tell us, what full rocker board you used. Not all rockers are created equal
Certainly they aren't all the same, but I was on a 2011 Ride Rapture, which would seem to be exactly the type of board the guy at the shop was suggesting he get. If I were riding park I might have kept the board, but for an all-mountain intermediate I found it was total crap. Given that the OP has been learning on a cambered rental board, seems pretty happy with his progress on that and has no interest in park, I doubt he'd like the ride of a cheaper, full-rocker board aimed at beginners.