Spheres and circles in my maps are generally created using WorldEdit, simply because it's the easiest way to create them. Pretty much the same goes for hexagons and other geometrical shapes used in this and other worlds I created. I often use a sphere or a circle as a base, pick the points I want to have as the corners of the shape and connect them making use of the line command in WorldEdit. In the end I just delete everything of the shape I don't need and that's basically all there is to it. I hope this helps ^^

As I told someone else just a short time ago this map was created in an older version. I just downloaded the world from the mirror (to get the same results) and fired it up in 1.8.8. Although some of the leaves seem to be vanishing due to random ticks the map generally loads in after a short time. If you are using a more recent version you might get unexpected designs but this can happen if you update over too many versions at once. I haven't tested it but you could try making smaller steps to get the world up to a newer version, load it in 1.8.8, then in 1.10, in 1.12 and then in 1.15 or something like this, but I'd just recommend using 1.8.8, the world was created in that version so no blocks will get corrupted and just to explore it that will probably be enough.

Hmm... If I remember that correctly it would have been 1.8.8. It wasn't the newest version as I created it, but the only one that had both World Edit and Voxel Sniper available.