From what I understand, the precast family you have will not work with the split-segmentation function from the precast tab. Again from what I understand, the way this function work is that this function 'splits' the structural floor into parts which are defined by the precast slab family. If you open the precast family file, they are setup as a generic model with extrusions while the slab profiles (the cuts) are defined under the profile tab (in the structural precast folder.
The way you setup your precast family is quite different compare to the Revit's precast slab family setup. The Revit 2021 setup for the split-segmentation function is quite rigid at the moment which is for precast hollow core slab or precast solid slab. The waffle slab that you are showing will be quite difficult to be adjusted to fit into this function.
The easiest solution that I can suggest for your is to make to make them into a structural beam. You can either make the whole slab a beam or make each form space and half a rib to be a beam, and make them into a beam system. Another slightly complicated (really meant more work for you) but better approach that I find would be easier to work with and also adjustable is from this video https://youtu.be/8ye6etopMj0