We report on a comparative study of the elastic strain relaxation in InAs(Sb)/In(Ga,Al)As heterostructures grown by molecular beam epitaxy on GaAs substrates via InAlAs metamorphic buffer layer (MBL) with and without a highly strained 5 nm-thick GaAs insertion. Reflection high energy electron diffraction was used for in situ monitoring an in-plane lattice parameter of the epitaxial layers during growth. As a result, critical thickness of the initial stage of the InAlAs metamorphic buffer layer as well as its corresponding composition were determined for the structures with different position of the GaAs insertion within the MBL. The structure with 5 nm-thick GaAs layer inserted in the InAlAs MBL directly after achieving the In content of 37 mol.% demonstrates the highest room temperature photoluminescence at a wavelength of about 3.5 μm.