Worked on this for three days. Pictured above, that was a desert. I dug up and replaced the surface piece-by-piece with dirt, and a single block of mycelium which spread across the land. Then planted the mushrooms, then used bone meal to turn some of the mushrooms into giant mushrooms.

Underneath is a zoomed out view. You can see the edge of the server’s spawn town in the bottom right-hoof corner.