RE: 6.5 sail for gusty lake sailing
Yes, you've hit on the main issue. Width!
Up to around 20 knots, boards 75 cm -100 cm wide
work really well, and have actually "opened up" the entire
wind spectrum from around 7-8 knots up to around 12 knots
to windsurfing. Short boards didn't plane much below 12 knots until the wider boards (like your original GO) were designed.
But, at around 20 knots, (unless you are a Formula sailor and racing
or training for races) narrower boards with modern improved rockerlines are so much more versatile and smoother to sail.
The width you like so much soon becomes "the width that makes control a limiting issue.
Sure, if you have 25 knots, on really flat water, or somewhere like the event site in the Gorge where you are just sailing back and forth across the river and only need to go across the river until the chop and swell gets uncomfortable (or unmanageable) your 5.5 m2 rig and a 46-52 cm fin will still be lots of fun.
But if you have to sail in real world 22 knot + conditions, with big chop and large wind driven swells that may not align so that you can sail "across them" sailing a wide board like the original GO (83 cm wide) soon becomes less than fun.
Something smaller and narrower like a small Carve, S-Type, Isonic,
or Kombat just makes things easier and more manageable as well as alot more fun.
If you feel you don't have the skills for such a small board, I think you will find that you actually do have them and that the wider board is really something you no longer "have to have".
The GO 129 is still too wide for 22 knots.
Your "I love the added stability" is not really an issue because at 20+ knots, with a 5.5 or smaller rig, you have plenty of power to make waterstarting really easy, and the modern freeride and slalom boards that are designed for this wind strength have plenty of stability once they are planing.
You will learn the skills you need to sail the smaller board very quickly
and will soon be going faster, jumping higher and working on your carving jibes.
Hope this helps,