Yeah, these covey an “older model” look very well, and certainly below the Toa in terms of power.
While I’m a little too busy to give my take on these, I can say that what I was going for in my other Matoran designs was insinuating clothes through the patterns and colors in the matoran. The fire matoran looked like they were waring armor and the ice matoran looked like they were wearing pelts and leather. While it makes it more difficult for the minifigs to combine with each other, it made them feel a little more in their own element. You could try doing the same thing with a little more subtlety than I did, but you don’t really need to change your designs much for that.
As for the heads, they do look skeletal, making them pretty creepy. What I tried to do when I did my take on the heads was give them facial features behind the mask, like different shaped eyebrows, noses, cheeks, etc. Var’s designs revised and elaborated on them. While it doesn’t make too much sense for robots, and takes some time to design each idividual, it can go a long way for giving them a personality.
My last note is that it’s a bit difficult to tell what different genders would look like. But it doesn’t seem like that’s what you’re going for right now.