Hello Martin,
You need to configure STMS using Extended Transport Control Using Client Dependent Transport Routes. If you are using client in one of the transport routes you must use clients for all routes.
Firstly /SOD_DEV/ target group is okey. This group will be between AED and AEC clients. ZAEC and SAP will be transport layers. All workbench and customizings will use these layers.
You donot need to create extra transport layer.
Between AED and AEQ you need to define target groups using clients. Another groups for AEQ and AEP.
Here is an example transport routes.
Regards,
Yuksel AKCINAR