Yes, footstrap position (inboard or outboard) can affect your control, but not perhaps in the way you are thinking.
Moving the footstraps outboard does basically 2 things.
It gives your foot and ankle more "leverage" to control the attitude (side to side or "roll" angle) of your board and fin.
It places your weight more outboard off the centerline of the board and if you aren't getting your weight totally committed to the harness lines and rig, having the footstraps outboard can lead to some control issues. If you are getting all your weight off your feet and onto the rig, then it becomes pretty easy control the roll attitude of the board as you can do that with better leverage, and virtually no weight on the tail of the board, so your ankles have the ability to hold the board at the attitude that gives the best control, and when you get enough TOW (Time On the Water) you soon find the board's attitude can be controlled very precisely even over the chop.
Having the footstraps inboard kinda does the opposite, and for your smaller boards (esp. for chop hops and jumping maneuvers) having the footstraps in the center position can be used to minimize the "leverage" giving better control on the narrower board with the smaller fin. Landing a chop hop or other "jump" in the outboard footstraps, on a really small board with a very small fin can be more difficult because you don't need the extra leverage, and you also don't really want your weight applied very mch off the centerline.
So, you need to adjust the positions of your rear footstrap (s) to suit the conditions, the board, and what sort of sailing you intend to be doing.
The flatter the water and the more "back and forth" (B&F) slalom sailing you are doing, the more you need the footstraps outboard.
The rougher the water, the more you are jumping and landing, and the more B&J (Bump and Jump) type sailing you are doing, the more desireable it is to have a singe footstrap over the centerline.
It sounds like most of your "spinout" issues occur when you start to head upwind.
Be sure that when you change course to more upwind, you do so gradually and when you roll the board and apply more pressure that you do it smoothly and with a steadily increasing pressure until you get the course you want.
Also, more time on the water is going to "teach you" how to "feel the fin" and become much more sensitive to when it's beginning to let loose and "spin out". In essence, you learn when you have the max. pressure and angle the fin can withstand (at a given speed) vs when you've applied a bit too much and the fin begins to "let loose".
Then you can begin to "ease off" when you get near the max. load and angle and keep your fin solid, but right on the verge of spinning out.
Try different fins.
Some let go with a bang, all at once. Normally this would be with larger more vertical fins.
Some fins never really seem to "hook up" solidly, and are a bit "loose" all the time. These normally are smaller very curvy planform wave and bump and jump fins. The "looseness" has been designed into these fins on purpose, so the board will be very loose and slashy, but they don't work very well for B&F straightline slalom sailing or fast speed sailing.
Actually, when I read your reply and you said your board was "dancing" a bit, I found that very encouraging.
When the board "dances" a bit, that usually means it's sailing fast, and on the minimum sustainable waterline length.
Now we just need to send Thomas to "dance class" so he can learn to dance to the same tune.
That's probably the big "secret" to solving your spin out issues. More time on the water so you become more sensitive to when your fins are going to let loose, and how to moderate the pressure and angle (with your feet and ankles) to keep the fin "right on the edge".
Hope this helps,