It would seem that you are really pushing the limits of the GO 144 here.
If you truly are sailing in a solid 20 knots of wind, the 5.8 sail size is about right,
but your board is getting too big.
In order to "calm it down" I'd suggest you work on getting into the back footstraps, with the footstraps fully outboard as this will give you much better control (once you get used to the differences between all the way back and outboard and your intermediate positon.
The stock 50 cm fin is almost too large for a 5.8 m2 rig in 20 knots.
Perhaps a smaller 42-46 cm fin would be better for this windspeed.
I understand that you found it difficult to get into the rear footstrap with the footstraps all the way back and outboard, but once you've done it enough it will become 2nd nature just like the intermediate straps seem now.
You cannot get the full performance from the GO 144 sailing it in the intermediate FS position. You simply do not have the control over fin attitude that will give you full speed with full control.
I do not understand your issues with the board going downwind if you fully load the sail.
Are you getting the foot of the sail all the way down on the deck of your board by raking it back fully? If you keep the rig standing up too straight (not raked back) then the sail will tend to pull you downwind.
The reason you are getting so much back foot pressure is that your foot is so far forward of the fin. When you get into the rear and outboard footstrap, you will find you have much better control over the rail to rail (tip) of your board and can steer more easily by lifting slightly with your front foot ans you push across the top of the fin with your back foot. This doesn't happen if you are not committing to your harness and have most of your weight "suspended" from your rig.
My guess is that you are at the "intermediate" stage where going faster is not yet comfortable, but I can assure you there's alot more speed in the GO 144 as soon as you get everything tuned.
Also, where are you running your mast foot (relative to the mast slot in your board), in the middle..... forward.... back?
Hope this helps,