"It's my understanding that AE will load balance across links, essentially behaving in an active/active state, where as reth traffic is more active/passive behaviour. " => not 100% accurate
I assume you want the SRX to function active/passive. If you want active/active, you need different config and then you also need to make sure the return traffic hits the right SRX; most do NOT configure this way.
The 2 x AE's on the EX4550 do NOT load-balance (right term is load-share). They will act as 2 separte links. By configuring them as AE's with single links, you have made them easier in the future to add in additional links for additional bandwidth, but they still work independant of each other. Load-sharing only occurs within a single AE, not between AE's.
The SRX cluster will control on which AE traffic is sent - only one member of a Reth will be active at any moment in time, associated with the active SRX. So even though the 2 x AE's on the EX4550 could function active/active, and link will be up on both, there will be limited learning on the inactive AE, which will force all traffic down the active AE.
See this link for details - https://kb.juniper.net/library/CUSTOMERSERVICE/GLOBAL_JTAC/NT260/SRX_HA_Deployment_Guide.pdf
Bottom of pages 10 and 11 state:
3 The secondary interface itself will actually still pass traffic, but there will be no flow processing for it. I.e., it is flow processing that stops the traffic. One example of traffic that can still go through the interface is ARP requests received on the secondary node interface, which will be sent to the active RE. The response to the ARP requests will be sent out on the primary node interface. Another example is IP Monitoring, which can send pings from the secondary node as well, to test connectivity.
I assume what you are trying to do is what is depicted on page 16 of the deployment guide, except the AE's from the switch (EX4550) only have a single link in them. Since SRX Clustering has not changed in a long time, the fact that the deployment guide is dated 2014 should not be an issue.
Here is a link that talks about supported/non-supported LACP configurations for SRX Clusters - https://kb.juniper.net/InfoCenter/index?page=content&id=KB22474
Your current config should work as you hace created 2 x AE's on the EX4550s (I believe). It is same as supported config example above but with 2nd interface on each AE being down.
I also found this later guide - https://www.juniper.net/documentation/en_US/release-independent/nce/information-products/pathway-pages/nce/nce0092-chassis-cluster-srx-configuring.pdf (contains nothing more than original HA/Cluster guide).
Hopefully this helps you.