Your spinout may be because you have too much weight on your back foot. You cant get away with that like a bigger board & fin.
Dont move the front footstrap forward unless you have control issues. All it will do is increase the waterline and make the board more sticky. You want to be riding the board off the fin. For flat water freeride, straps at the back, mast track a bit further forward than for wave sailing. I think Starboard now have this marked in different colours on their Kode Freewaves.
If you do get a bigger fin, dont get a wave/freewave fin. Too much area and drag. A narrower more upright slalom type fin would be better. Does your board have a US box or powerbox? A US box will greatly reduce the options available, especially secondhand.
I used 5.6m & 6.5m 3 cam freerace sails (Naish Redline) on my Exocet Cross 90l. I think that board is more freeride than your Starboard FSW, its doesnt have any tail rocker. Does the Kode have much tail rocker? I think you need tail width and thicker sharper rails for cammed sails, look at the back of a small slalom board.
With the 5.6m sail I used 25cm and Select 28cm OE fin (similar to the 29cm Select Edge). The 25cm Select was a bit small, but it was very cheap at the time.
With the 6.5m a 32cm F-Hot Blade or the 28cm. The F-Hot was very narrow compared to a modern F-Hot.
For my current Cross 94l, with a 6.5m 2 cam Ezzy Lion I use the 29cm Select Edge on flat water and the OE Exocet 30cm fin when on the sea. I'm 78-80kg. I dropped the 3 cam sails as they were hard work. To make having them worthwhile you have to be well powered up all the time. I was on the 5.6m when I would have used my 4.7m wave sail, or 6.5m when I would have used a 5.2m.
TBH, I think you would be better off with a 5.5m 5 or 6 batten non cammed sail with your Starboard Kode. For flat water, more of a freeride than wave sail.
I like the old Mistral 25cm fin, its got a bit of curve and rake, low area, for blasting around with 4.5/5.2m. I damaged the tip so bought the Black Project 26cm when I changed boards. I've since repaired the tip as its a good fin. The 29cm Select Edge for 6m & 6.5m on flat water. The wider 30cm, 26cm and 24cm are nice and forgiving on the sea as they have more area, but the others are better for flat water and sailing faster. Changing fins can alter the feel of the board a lot. Just buy secondhand or new ones when they are on sale.