A difficult question. FOr the Small bord I think Quad 76 or even 81 will probably work great. To and light wind float, you might wanna go with the 81. Dimensions are very similar. The 81 has a bit more drawn out outline which might even be good for fast waves in SA. I dont think you will feel its too big. But the 76 is of course very nice too, with a more compact feel. And even the 76 wil surely get you going earlier and feel more reactive than the EVO 75.
The smaller quads are EVO based, so if you don't like EVOs that it's its not certain you will like them alltough they do feel a bit different. And the 66/70 EVOs to me always felt faster and more reactive than the 75 you have. And the Acid 73 you have was actually a tweak of the EVO 70 shape from the start so...
But if you want to play more safe, the Kodes are also awesome boards. I do think the 68 will be a bit small. At 80 kilos I'm sure the 74 will handle both high wind and fast waves. Personally I think the current generation 74 is much better when the going gets tough, than the Acid 73 you have now. So unless you feel the A73 is totally too big I think the Kode 74 is the natural successor.
If you end up going for a small quad also, I can't really say which size since I have not tested the in high winds yet. The 71 should normally suffice, but the 66 will definitely feel even more reactive at high speed and with the extra fin drive of the quads, I think it will carry your weight better than a comparable single fin (aka EVO 66) would provided you sail where you can keep the speed up.
Where do you sail and when do you need to order? The best thing would surely be to demo one.