Interesting ideas. I agree that having a single slider doesn’t work so well with shells. If we put shells in the external organelles section then they are binary, there is only one thickness rather than a slider.
I’m open to options on this.
One possibility is to have a 2D selector, with the rigidity on the x axis (from soft -> normal -> double -> cell wall (both chitin and cellulose together)) and then have shells on the y axis (silica at the top -> none -> calcium carbonate at the bottom). That way you can have any kind of membrane and then put any thickness of shell around it. Cell wall + thick shell would be the ultimate defensive strategy.
