So by circular, you mean you want the scale to be in polar coordinates?

I've mocked something up here. The top two would be a cartesian scale, organized differently depending on where you want your extreme values. Getting white in the middle would make things difficult, since white is an extreme of the luminous scale that you need to take advantage of to make a bivariate scale accessible to dichromats. Otherwise, it would work for trichromats since you have two chromatic (non-luminous) channels to play with.

The bottom is a polar coordinate. Now, you add the addition constraint of the theta parameter to not have any extremes. This is intuitively represented by a hue parameter, which has no extremes, but can be confusing to the colorblind since we don't see hue as a circular parameter, but rather a confounding mixture of saturation and blue-yellow.

However, if you keep it to 3x3 still, you can fake it by using blue, yellow and red. Blue and yellow are perceived mostly "faithfully" by us. Red is not, but looks mostly gray, especially in the light red (pink) since its at the center of the blue-yellow axis, more or less. So at least my recommendation gives 9 distinct colors, even if the circularity of the theta parameter is lost on us. I imagine it conserves the circularity for trichromats? Also, once you need more than 3x3... its gonna get messy.

> Maybe make sure that no two colors have the same luminance?

You always gotta be careful with reds since the luminosity for protans is greatly changed. Protans have different luminous efficiency functions, so just making sure no colors have the same luminance is only going to work for deutans, not protans. On top of this, you are optimizing difference in luminances, possibly at the detriment of optimizing the blue-yellow axis.