Switcher discontinuous mode - potential EMI problem
DCM is not necessarily less efficient --it depends on the controller.

If it is possible to raise the 5 V a little, you could add a linear regulator, limited to 1 mA in parallel with the SMPS. If the linear regulator regulates at (say) 5.2 V, the SMPS will see that VOUT is OK, and will fully switch off -- no switching cycles. When the load exceeds the 1 mA capability of the linear regulator, the SMPS will take over.

in addition, the inductor node will ring in DCM when the driver turns off -- put ~ 1kohm in parallel with the inductor to dampen this ringing. It will have a slight effect on efficiency though.