This has come up a few times for 2d (Finding centroid of wavy enclosed curved pieces - #4 by DanielPiker), and I expect an extension of the same approach to 3d would be the best way to approach this.
Anyway, here’s a way you could use fields to approach this - I don’t think it’s a very efficient way to solve the problem but maybe fun to see.
insphere.gh (154.6 KB)
Insetting is simply a matter of adding a constant to the distance field. As this increases the volume will eventually decrease to nothing, and this point is the furthest inside the shape and the center of the maximal insphere.