Where are you placing your rear foot when you go for the front footstrap?
Try to place it directly over the fore and aft centerline of the board.
Also, place your foot (try a few different positions) in a spot that does not
cause the nose to pop up.
You will have to find the spot where the nose lifts slightly to change the planing surfaces
to positive, but not enough to cause the nose to "pop up" which will put too much positive angle on the planing surfaces and cause your board to actually slow down as the drag will increase significantly.
Also, when you start putting your weight on your back foot, do so very gradually.
I know most sailors who are "stuck" just before planing want it to happen quickly, so they
tend to want to move their weight quickly. Since you are a big guy, you will have to be
extra careful and really "finesse" the fore and aft trim.
Also, wait a little longer as more speed is your best friend here. More speed creates more lift on your planing surfaces, improves the fore and aft stability, and makes
transitioning from "semi-planing" to fully planing much esier.
So, wait awhile and allow your board to gain a bit more speed.
Also check that you are not putting any weight on that front foot.
(Are you crazy, Roger, how can I not put any weight on my front foot when I get ready
to move my back foot?)
Well, after many years of careful analysis of what I do, and watching many of the pro level sailors, I've found that you really only need 2 points of contact with your board.
When you are not hooked in..... and at sub planing speeds, yes you have weight on both feet.
But, as soon as you hook in (if you have a large enough sail for the windspeed) you
can have "two point contact" while your front foot is off the board getting into the front footstrap. Huh??????? Yes, you have the rear foot, correctly placed over the centerline of the board so you can steer, and the mast foot (which pushes the front of the board in the direction you want to go).
You can practice this...... get solidly hooked in, get your rear foot correctly placed, and
try waving your front foot over and around the front footstrap. Delicate feat of balance
..... yes, but one you need to learn.
Since, you are now waving your front foot around, and all of your weight is being supported by the harness up through the rig and down to the mast foot, go ahead and put your front foot into the strap, BUT KEEP ALL OF YOUR WEIGHT ON THE RIG!
When you are balanced, and your board has PROGRESSIVELY gained enough speed (take your time here) then slip your rear foot into the back footstrap.
Also, as an aid to "directional stability" in this transition to planing you might want to try a big weed fin (53 cm is about as big as they make). Big weed fins have a lot of fore and aft "directionality" and this may help with your "wobbles" until you are beyond this stage.
Formula fins are 70 cm.... but Gorge Fin Co. has made some larger fins.
Right now, I don't think you need a larger fin as you are not up to enough speed (still marginally planing, right) where you can learn to "ride the fin".
At some point, at your size (300 lbs./136.07 Kg/21.4 stone) you will probably want a larger fin for your GO 175.
A 60-70 cm fin might help you now, but I think the suggested weed fin may have more longitudinal directionality.
Let us know how you get on.
1/ More speed before you try to transition.
2/ Better placement of the rear foot.
3/ Take your time and be sure to get as much of your weight as possible
on the rig. (Lean back, do not squat.... hook the top of the rig upwind a bit
to help the rig lift and support your weight).
Hope this helps,