You have gotten some good advice above. Since it is mainly flat water and after all not som many super windy days, I would not go for a smaller board than the 74 so I think your original question 74 vs 80 is a good one. The two boards are fairly similar and also the 80 feel pretty small and is controllable in chop. It is mostly when you come to the point where the wind start to blow the board around that you will feel the extra size of the 80 in a negative way. I would say that this is somewhere around powered up 4.2. So from that stage and obviously with 3.7 the smaller size of the 74 will make the sailing ore comfy. Conversely, the 74 easily handles 5.0 and isn't really significantly slower to plane with a 5.0 than the 80. The difference is more in "ease of use" The 80 will glide a bit longer through a jibe, ie keep speed better, and the wider platform will also forgive more mistakes in maneuvers.
So in a way, the 80 will be a more natural step down from the gw83, which I believe is a Naish traditional kind of "hard" shape (correct me if I'm wrong). The K80 will for sure feel much with 4.2 and 3.7 than the GW83.
So, if you feel the gw83 is in the verge of being acceptable with small sails and if you think it feels good in size with a 5.0, I would say the kode 80 might be it.
But if you really struggle with the gw83 with small sails and maybe feel it is unnesessarily big with a 5.0, then you should look for the Kode 74.
Also, the more used you are with low volume boards from before, the more the recommendation will shift towards, the 74.
The back ground of this recommendation as a whole is that it is not nice to buy a new board and find it too small and that it gets left in that car in marginal conditions because it just feel to cumbersome to sail. The 74 and 80 are two boards in the same class and these kinds of boards are super fun to sail. So ideally, you should be on it as much as possible, not leaving it only for nuking days. The 74 will play this role too, despite being a bit smaller and in the end it will indeed be an overall better fit for 3.7-5.0. But it will only work if YOU feel comfortable on it.