As far as I can tell, there is no way to access the midpoint of an arc. This also means that there is no way to dimension the distance between the midpoint of an arc and other sketch geometry. Please add this ability in a future release.
As a general point, I find that Shapr3D is lacking some basic sketching tools like this. Another example would be the current inability to align points as a constraint. Other CAD software makers have gone on to add all kinds of other features, without finishing sketching / drawing tools comprehensively. I’m really hoping Shapr3D doesn’t follow this same path.