If you're having trouble with it turning upwind put the mast track forward a bit, move your weight forward, stand in the centre of the board keeping the board flat, DON'T sink the windward rail. Try this first.

I tried thrusters with the original (crap) Bic plastic fin and it did help a bit.
Then I bought a fiberglass 10" dolphin fin and it works great without the thrusters.
Better directional control and less drag when paddling also
I run a 8m sail and use the centreboard in light winds (5kn) and upwind,
but reaching in 10kn and over the bic 11'6 planes nicely with just the 10" FG Dolphin

The side boxes should be FCS , 2 small grub screws hold the fins in contact 2 tabs on the fins. Commonly called FCS originals, newer FCS and some variations are different fitment.
I had a BIC something that I installed a mast track ,10'6", his may have a screw and not therefore not adjustable.
The fins supplied are bottom shelve. That's complimentary .
K4 have a SUP Racing fin, in 2 sizes last I looked. I used a 22cm.
i always used the sides. I would say 10-12cm be close match.
This could be enhanced with the sail size , sailors weight etc . Bla bla bla. Amount of toe in on the sides would influence my pick on the thrusters.
K4 , have the FCS fins , they are asymmetrical also.

i think in 3 years the BIC only planed once. Replaced with a AHD Sea Lion, brilliant.
the BIC IMO , is not a performance Wind Sup.
plenty of FCS , that's the brand of fin and also the fin base, exist. Also plenty of US box fins for the center.
the K4 SUP racing fin and the K4 sides, would be a improvement and very low cost.

The Bic 11 6 and 10 6 have a standard windsurfer mast track.




As has been said before putting your mast track forward might help but I think the problem is getting your weight distribution right you might be slightly titling the rail causing you to go upwind usually with a combination of foot steering and sail steering turning your board into a reach the more you starts to move the water flowing over the fin will make if more effective enabling it to track in the desired direction. Turning into the wind is a common problem for people learning and you just have to learn to correct for it until you have enough momentum for the fin to work.

How does the position of the centre fin in the fin box- forward or backward impact how the board performs ?

I managed to obtain on gumtree a "windsurf" shaped fin - more upright (about 17 inch). More area for the 11.6 foot sup I currently sail RATHER than the 12.5 inch "surf fin" I had.
Still have the two side fins as the SUP is tri-fin. The more fins the better - More to push against.
Also Fin placement makes a difference - put the fin as forward as possible. Board heads higher upwind.
Given - here you can not use large sails. Running a 4.8 to 5.2 with this maximum.