FE Strength Reduction Method for a Slope already have LEM FoS < 1

Aside from the type of analysis - since you showed the slope strata, etc., it would be nice if you included the soil types and strength/density for each of the strata and location of groundwater table. You say that you are building in stages . . . then the base layer (yellow) I suppose would be soft clay. With the first stage going on, how long before the second stage? During that time the lower clay layer will see an increase in strength due to the consolidation due to the first stage (and subsequently to the later stages). Are you taking this into account with the succeeding stages in your LEM? We built RE retwalls on 6 m of very soft clay in stages (with wick drains to speed up consolidatin) successfully taking into account the increase in the soft clay's increase in strength - 11 m high wall settled 1100 mm during construction.

When using the SRM you apply a factored shear strength based on the shear strength of the soil divided by a strength reduction factor (SRF). For soils defined using cohesion only: cohesionfactored = cohesion/SRF. Now you can see that if you chose a SRF=0.7 the slope may be marginally stable as the factored shear strength is now greater than the true shear strength of the soil. You would say that the factor of safety of the slope equals the SRF associated with instability which is commonly defined at continuing displacement or non-convergence of the model. For your own information you might track displacement vs. SRF by choosing a point in the model to track as you continue to increase the trial SRF from some safe value towards instability.