I've ridden most of the trail centres in Wales.
I'd suggest either sticking to the South or North only - the roads through the middle of Wales are slow going.
If staying South I'd probably go for Afan (W2 trail), BPW and Cwmcarn (2 x Twrch trail).
Cwmcarn would be on the way back to Soton, the other two aren't that far apart.
You could also head over to Brechfa which is worth the trip (though in my opinion best when it has been dry).
Best to get Brechfa one done on the 1st or 2nd day as the drive back to Soton from there would be a long one.
If you were going for North, then Penmachno/Marin can be done in a day (10 minutes drive to get from one to the other), a day at CYB (avoid the Tarw trail - it's not good - go for the Beast/MBR/Dragons back instead) then Llandegla on the way out.
Staying somewhere near CYB or Betws-y-coed would be good if you went for N Wales.