Hi Matthew,
Yes, I live on an island in the Mamanuca group. I would advise you against this area. There are Gts, but the fishing is patchy (I still love it, though), and it is expensive.
This is a generalisation, but there are four areas of Fiji which combine good Gt fishing with great diving-
Kadavu- deep south. This is where many forum members have fished with great success, and perhaps they can offer advice. I do not know this area but it will be expensive.
Beqa Lagoon. South of Viti Levu. Gts quite pressured but great diving and easy to get to. Affordable accommodation on Coral Coast/Pacific Harbour.
Taveuni. North East. Another area visited by forum members. World famous diving in the Somosomo Straights.
Vatu-I-Ra. Between Viti Levu and Vanua Levu. Arguably the best diving in Fiji and terrific fishing.
Fiji is not a particularly cheap place to visit any more. I think I would combine going to an established resort (such as Volivoli) where you can be certain of good service, and then taking a chance on a backpacker type resort, and trying to set up some fishing with a local. One I want to go back to is Takalana, on eastern Viti Levu. But please ensure you release any Gts.
Hope this helps.