Maybe Transition from Early to Late Multicellular could be done by making your single-cell hexes 4-cell, 9-cell, 16-cell etc., You would basically have option to multiply all cells in your organisms to this next hex level for 100 MPs, and you would get to late multicell when you have 1000 cells. When moving from 1 to 1000 cell organisms game will turn from 2D to 3D as well. At end of transition, you will get choice to make your organism clip by itself, like circle rotating 90 degrees and creating sphere, to create radial/spherical symmetry organism. Or you could just turn your organism into torso made of metaballs, which would start appearing as you get more cells per hex. This could be better than current “20 cells to late multicellular” solution, as it would not allow cell types with 1 cell representative to become it’s own tissue.