I'm a beginner & only go to Tweed Heads a few times a year so probably wise to wait for one of the pros and/or locals to reply... but anyway:

I've seen Currumbin Alley teeming with windsurfers in any wind other than a bolt eastely or westerly, could try that?

I've also seen some nut-bars sailing at the mouth of the Tweed River in the right wind ... amongst the trawlers, pleasure-boaters, sharks, rocks etc

There's the Broad-Water but probably too boring unless you're like me & just learning to water-start & get into the footstraps.

Hope this helps - I invite the comments of anyone who actually knows what they're talking about & isn't just winging it!

Broadwater - boring????.... mate I reckon the train is one of the best spots in Oz blasting along the sandbank on dead flat water, I would recomend the drive up there for sure