RE: That pesky rear footstrap
OK, a 50 cm weed fin can easily have more area than the stock fin, so you may have been a little overfinned.
On the other hand, a 6.3 m2 rig (a recreational sail, not race or free race perhaps) is a very small sail size for a board as wide as the F-Type 158.
And, with the footstraps inboard, you probably aren't going to have the leverage (with your feet and legs) to control even a 50 cm weedfin.
We need a little more information here.
What brand and type of 50 cm weed fin.
What type and model sail.
How windy was it......
How large was the chop.....
Sounds like you may be ready to move your footstraps outboard, and maybe take a slightly smaller or narrower board.
The F-Types do accelerate very quickly, so you may need to learn to
sheet in and rake your rig back a little more slowly to let that "acceleration" build up at a rate that allows you to get both your feet into the footstraps before the board gets going too fast to do so.
If you stick your front foot in, hook in, and then simply "crank" the rig in and rake it back, the F-Type 158 is going to do exactly what you are telling it to do. It will practically accelerate right out from under you.
So, take it a little slower, once you have that front foot into the strap. hook in, then work your back foot into the rear footstrap before you really get your rig fully sheeted in and raked back.
Also, it sounds like you are fully ready for the footstraps to be moved outboard.
That will give you much better leverage to control the fin.
Hope this helps,