So on the current 70cm race boards is it best to go slow and point as high as possible or head down a bit to achieve the best VMG?
u can't make such generalizations. Depends on the wind speed. For instance here's a polar made by 76 trombones I think some random blogger but it's a good example
Take a look at the red 20 knot line. Right about 30 degrees it begins to steeply slope in, which means, you do not want to tack upwind at 30 degrees. You also can notice at 20 knots that there is not a lot of difference between 42 degrees and 50 degrees, so you can see that you want this particular boat riding about 42 maybe 43 or 44 degrees when going upwind. Going downwind you see that the curve drops off steeply at 150 so you don't want more angle than that downwind. On the red curve you get a few knots of speed going from 90 to 110, but then from 110 to 150 it's pretty level. For this particular boat at 15 knots you would go directly downwind to the mark.
Now look at the 6 knot bright green curve. This one slopes much harder at 45 degrees so here you would want an angle of 45 degrees maybe 46 or 47 not 42. And also you want it directly downwind for that wind speed on the downwind leg. For 30 knots though, you might want the boat at 100 degrees not 90 when racing downwind you can tell by looking at the curve.
And no we can not make a polar for your kite and board because they don't exist you have to make one for yourself.
You can make a polar based either on true wind speed or apparent wind whichever seems to work best for you.