One other suggestion (if you REALLY want STP type design) would be to instead use RTG for EX4550 to EX4300. See:
You should still run RSTP (default) on all user ports with Edge setting if you like. RTG would be just for switch to switch connection. With RTG (IMHO) config is easier and you get one less control protocol out of your network. It times of bad network operation, the less control protocols you are running, the less likely you will have complete meltdown.
Just FYI