Keep it simple as follows:
Main:
You can simply edit a materials name list, no values, only your own expression. You can also choose one material name as currently active.
This selection stays until you choose another one. No connection to Solidworks materials or whatever, because we keep this simple.
Speed & Feed:
In the Speed & Feed tap you can select the material name and input your desired values. Check above picture.
When I select tools then the values of the active material (see picture main) is taken. No link of the values, no "live update" because we keep it simple.
If the material is not existing in the Speed & Feed a message shows and ask what material you want to use instead.
Of course I can copy a tool and put it in a library for different materials. But if something changes I have to manage and modify many tools that are virtually the same.