Howdy Larson! And Welcome!!!
You really won't need the extra thickness. Half Inch is way overkill - it will end up weighing the same as all those other boats that you didn't buy the plans for!
12mm will be Eight times stiffer than 6mm. Not double - but eight! and the chines - the only place it can possibly break will be the same size anyhow (but they won't break either)
Nice thing about the Goat is that built as per the plans it is a reasonable weight for two adults to handle on the shore. Increase much at all an it all becomes a big chore.
And of course the sailing performance won't be the same either.
If it makes the difference between building or not building the boat then go for the thick stuff - providing the rest is the plan spec. But if you want a thing less ordinary - build it as per the plan.
Best wishes
Michael