When you are doing anything like this the main thing to do is follow the manufacturer’s instructions otherwise you will void any warranty. So any advice I can give you should be taken under advisement.
If the instructions don’t specify that the tray needs to be bedded into mortar or mastic then you could put it straight onto a plywood base, it shouldn’t need a cement backer, as no water should get underneath. In fact that is the main goal in fitting a shower (stopping water getting underneath that is).
SBR is used to stop moisture being drawn into the board too quickly and aid tile adhesion, it is not a waterproofer. Unless you are planning to coat the ply or backer with adhesive there is no need for SBR.
Hope that helps.