To achieve the expected level of sensitivity of third-generation gravitational-wave (GW) observatories, more accurate and sensitive instruments than those of the second generation must be used to reduce all sources of noises. Amongst them, one of the most relevant is seismic noise, which will require the development of a better isolation system, especially at low frequencies (below 10 Hz), the operation of large cryogenic silicon mirrors, and the improvement of optical wavelength readouts. In this framework, this article presents the activities of the E-TEST (Einstein Telescope Euregio Meuse-Rhine Site & Technology) to develop and test new key technologies for the next generation of GW observatories. A compact isolator system for a large silicon mirror (100 kg) at low frequency ( < 10 Hz) is proposed. The design of the isolator allows the overall height of the isolation system to be significantly compact and also suppresses seismic noise at low frequencies. To minimize the effect of thermal noise, the isolation system is provided with a 100 kg silicon mirror which is suspended in a vacuum chamber at cryogenic temperature (25-40 K). To achieve this temperature without inducing vibrations to the mirror, a radiation-based cooling strategy is employed. In addition, cryogenic sensors and electronics are being developed as part of the E-TEST to detect vibrational motion in the penultimate cryogenic stage. Since the commonly used silicon material is not transparent below the wavelengths typically used in the 1 µm range for GW detectors, new optical components and lasers must be developed in the range above 1500 nm to reduce absorption and scattering losses. Therefore, solid-state and fiber lasers with a wavelength of 2090 nm, matching high-efficiency photodiodes, and low-noise crystalline coatings are being developed. Accordingly, the key technologies provided by E-TEST serve crucially to reduce the limitations of the current generation of GW observatories and to determine the technical design for the next generation.