This study provides an in-depth TCAD-based analysis of how spacer engineering affects the performance of multi-channel nanosheet field-effect transistors (NSFETs) designed for high-performance applications. The study systematically evaluates single-k and dual-k spacer configurations using essential DC characteristics such as ID–VGS, ID–VDS, ON-current (ION), OFF-current (IOFF), switching ratio (SR), subthreshold swing (SS), drain-induced barrier lowering (DIBL), and threshold voltage (Vth). Among the single-k spacer materials investigated, HfO2 provides the most favorable electrical performance. For dual-k spacer engineering, the HfO2+SiO2 combination is identified as the optimum configuration, and further spacer-length optimization shows that a 4 nm/3 nm low-k/high-k arrangement yields the best switching characteristics. The optimized device is then examined under variations in gate length, nanosheet thickness, nanosheet separation, and nanosheet width through both DC and AC analyses. In addition, important analog/RF figures of merit, including gate capacitance, transconductance, output conductance, gain, cut-off frequency, intrinsic delay, gain frequency product, transconductance frequency product, and gain transconductance frequency product, are evaluated. The study provides a detailed assessment of how spacer selection, spacer-length optimization, and geometric tuning influence the electrostatic and analog/RF behavior of multi-channel NSFETs.