GaN power ICs on engineered substrates of Qromis substrate technology (QST^®) are promising for future power applications thanks to the reduced parasitics, thermally matched substrate of poly-AlN, high thermal conductivity, high mechanical yield in combination with thick GaN buffer layers. In this work, we will elaborate in detail on epitaxy, integration, and trench isolation. Electrical characterizations show that the GaN buffer bear a breakdown voltage of > 650 V under the criterion of 10 μA/mm² leakage current at 150 °C. The fabricated 36 mm power HEMTs with L_GD of 16 µm show a high threshold voltage of 3.1 V and a low OFF-state drain leakage of <1 µA/mm until 650 V. The horizontal trench isolation breakdown voltage exceeds 850 V. The device dispersion is well controlled within 20% over full temperature and bias range. Finally, GaN power ICs on this platform are demonstrated.