Thanks for the detailed description, that's really helpful! I think my arm position is properly below my shoulder, because with the easier exercise I first had trouble holding it because my elbow wasn't placed properly, so I learned to pay attention to that, but my problem may indeed be the tension in my feet now with the harder one.

With the easier variant the lower legs don't need to be tense, and when I tried the harder I didn't keep that much attention on that as on my core muscles. So when I try next I'll be more attentive to first building the tension including in the feet/legs, and see whether maybe the not slipping will follow (rather than lifting up and then trying to be straight and not to slip and wobble all over the place).