We report the full sub-Kelvin characterization of the BAUSCIA high-frequency gravitational-wave antenna, based on cryogenic Bulk Acoustic Wave resonators coupled to SQUID amplifiers for signal readout. The end-to-end performance analysis of the experimental chain, from the sensor to the DAQ, demonstrates that resonator thermal noise dominates the noise budget at temperatures well below 100 mK for all investigated modes. The measurements also verify proper thermalization of the resonator modes and rule out significant SQUID backaction on the strain sensor, confirming that the system operates close to its fundamental limits with comparable resonator thermal noise and additive readout noise. From this characterization, we derive projected strain sensitivities in the range 1--10 × 10⁻²¹ Hz^(-1/2) for several resonant frequencies between 5 and 20 MHz, with corresponding fundamental thermal limits in the range 0.7--1.4 × 10⁻²¹ Hz^(-1/2). Equipped with three resonators and synchronized to an absolute global time reference for multi-site coincidences, the antenna is ready to enter a pilot science run.