Coming from someone that has only been windsurfing for about 17 months this is what I found while learning(well still learning):
I found that Dorans is the best place to learn and is the safest place to sail in W/WNW/NW/WSW, followed closely by Cremorne lagoon in a Northerly or Southerly. On a NE it is def worth the effort to make the trip to Marion Bay on a high'ish tide, I think this is a great spot to learn, very safe and mostly shallow.
But recently we have been getting quiet a few SSW winds which don't come into Dorans (especially where its shallow) and even SW winds don't usually come right into the shallow water, only the far side as you said. Usually a SW comes in around the jetty in the top car park so you end up sailing towards the police academy, so when you see a SW direction you won't see to many people in the bottom car park sailing where it is shallow.
Mortimer bay is the spot to go in a SSW or SW (when not coming into Dorans at all) when the tide is high, you can still sail there on a low tide but it's a bit of a walk out and you wont be sailing in the shallow water, because its to shallow. Also if it's been raining and you don't have a 4WD it can be a bit hairy getting in as the track gets so muddy.
On a high tide and a SW wind Mortimer isn't a bad place to sail when you're learning as you can pretty much stay in the shallow water if you don't sail out to far, but I found it gets quiet choppy.
Once we start getting some good W/WNW winds you will love sailing at Dorans, because you just sail back and forth all in shallow water :) or seabreezes at the lagoon :) No one has been getting any good flat water sailing in recently, because the winds been bad! But fingers crossed this changes soon

Mortimer Bay: