Why run any form of STP at all? STP designs (outside of Cisco) went out 10+ years ago. Just change the 2 x 10GE connections from remote EX4550 to EX4550 in the code to an AE/LAG and you'll have active-active on both links, without need to STP.
With today's various other technologies, STP should really never be needed. Primary case (for Juniper) is interoperability with previously configured/designed Cisco networks. For EX to EX, you could also use default RSTP config to start with.