I spent 4 months last year in Fiji, mostly on the southern coast of Viti Levu (main island) and these are the places I wingfoiled:
1) Pacific Harbour, at the Pearl Resort beach to be exact. Had great sessions there. No waves but nice bumps that formed.

2) Wailoaloa beach, right opposite of the runway at Nadi airport .. check in at the Ramada hotel and go winging .. I had two great sessions there. No waves as your inside the huge reef, but nice bumps. Weather and water are super warm so it's a treat! Stay clear of the water plane port !! 😀
3) Maui Bay, closer to Sigatoka. The spot is very small. In general the Coral Coast offers little opportunity for wingfoil, unless you have a 40 cm mast at high tide. The Sigatoka River mouth you mention is part of the Sigatoka Dunes Natural Park. Shorebreak looked massive. The place is deserted. Needs some back up support. I think there is a nearby surf operator.

Drone footage here
Other options.North: You can go to the north side of the island but will cost you half a day to get there plus Fijian taxi costs especially right now are through the roof.
Namuto.. the paradise only in reach for the rich 😂😂 Actually the guys at Fiji Surf can boat you up to Tavarua and Namuto and you can play there. Will cost a couple of 100 USD. I had arranged the boat but no wind.
Ferry out: it's easy to take your gear onto the boats that go every days to the smaller islands. We were for a week at Plantation Island from where you can reach Namuto too. Is just there was no wind for me.
If you're going to stay a week:
- with noise and not so nice beach, choose Wailoaloa - but with daily access to the nearby marina (Denarau) that has dozens of daily boats to smaller islands.
- no noise and nice beach (but clear coral coast waters) go to Pacific Harbour
- Maui Bay for quiet, lots of clear coral coast waters, snorkling etc, and if the wind falls right then a session at the small Maui Bay spot.
Hit up the guys at
www.instagram.com/fijisurfco/ first to get info on a boat for the day, they operate near Denarau Port. I think that guys like Berndt Roediger link up with FSC to get their cloud break juice. Namuto is not only for the rich