The thickness is just a scale parameter when converting suface conductivity to permittivity. https://optics.ansys.com/hc/en-us/articles/360042244874-Graphene-surface-conductivity-material-model
So yes you can use other values for the thickness. The thickness does not change the Fermi level, but change the permittivity.
As you can see, for graphene, Lumerical provides two different types of material models, both are analytical. You can use either of them. You may need to compare with your experiment results and see which one is better for your device.
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