What you want to model is so complex it must be done via live analysis and measurement if you need viable data.
The general approach of using virtual springs to represent the stiffness of the linear guides is exactly what I would recommend.
Specification sheets for linear guides/bearings should list load vs deflection curves. You can extract a stiffness constant from such curves to input as the spring stiffness for each direction of loading.
To keep it simple I would use Normal stiffness only for each of the springs. This way each spring will represent one component of the linear bearing's stiffness (using 3 springs per 1 physical guide/bearing)