The objective of the proposed research is to advance frequency-modulated (FM) MEMS sensing by developing vacuum-free resonant pressure sensors for harsh environments and phononic frequency comb–based inertial sensors. FM sensing detects physical changes via shifts in resonant frequency, enabling high precision, fast response, and strong noise immunity. This work introduces a novel fabrication method for piezoelectric MEMS membrane resonators using die-to-die bonding and film transfer, creating a high-sensitivity absolute pressure sensor based on the squeeze-film effect, achieving a record sensitivity of 13.2 ppm/Pa from vacuum to 33 kPa while allowing precise cavity definition without sacrificial layers and seamless electronic integration. In parallel, strongly driven Aluminum Nitride and Aluminum Scandium Nitride resonators are used to generate mechanical frequency combs, with their dynamics modeled via Lagrangian mechanics and characterized through electrical and optical measurements to enable compact, high-performance inertial sensing. Future efforts will investigate comb phase coherence and the influence of external perturbations on comb spacing to further enhance sensing accuracy, stability, and multi-parameter detection.