I am you weight and restarted last year after a 20 year hiatus.
Daggerboard are not outdated. They are great on longboards. They allow you to go anywhere you want, even when the wind drops down. I sail on a lake, so I can launch from where I want, go quickly to a good windline using the daggeboard, blast around with the daggerboard up, and come back. Many intermediate and all beginners not using daggerboard here will choose the launch point very critically to make sure they will be able to sail very close to it.
The GO are optimized for planning. When on the plane, you don't use the daggerboard at all. On the GO windsurfer, the daggerboard will be used for to glide in low winds, or to go upwind for someone lacking the skills in medium wind. The glide will not be great because of the width of the board. The daggerboard is more or less a safety feature in my opinion.
There was a lot of improvement with the rigs in 20 years. They are more stable than they were, and dampen the gusts with their twisting leach. I restarted with my an old 6m2. I was overhelmed in 15 knots. My newer 7.5 m2 no-cam freeride sail (Ezzy) is much easier to use in that wind, and I don't change it until close to 20 knots. You should buy a similar sail. I am now looking for a 9.5 m2 sail to extend my planing range in low wind. My kids (30 and 45 kg) use 2.5 and 3.2 m2 sails.
With 21 knots, you will be planing fast and there will be some chop on the water. A large board will bounce around on the water, seemingly having a life of it's own. That's why smaller board are nice in these conditions.