I believe Walker Bros are going to be selling off a few wheels from the 1 August, might be worthwhile keeping an eye on their website.
From an aero perspective having the best aero rear disc isn’t really important, just try and get the stiffest, lightest you can afford. The FFWD is probably the best value and will fit most frames as it’s lenticular but not too wide. There are some good deals on the Mavics at the moment, if you want to spend more, perhaps they’re bringing out a new model.