To reduce high-amplitude startup overshoot, slow load-disturbance recovery, and phase-shift-command fluctuation in a phase-shifted full-bridge power ultrasonic generator, a constant-amplitude control method combining feedforward LADRC with RIME is proposed. At the 80% amplitude setpoint, the feedforward term is obtained from prototype steady-state calibration, while LADRC bandwidth parameterization reduces the tuning variables and RIME optimizes them offline. Five repeated prototype tests are conducted on a 40 kHz STM32-FPGA platform. Compared with PID, the proposed method reduces startup settling time from (783 ± 12) ms to (290 ± 5) ms, overshoot from (4.81 ± 0.16)% to (0.42 ± 0.03)%, load-disturbance recovery time from (690 ± 16) ms to (200 ± 5) ms, and the maximum voltage-feedback drop from (1 184 ± 25) cA to (411 ± 10) cA. Tests at 60%–80% operating points retain good disturbance-rejection performance. RIME is used only for offline tuning, requiring neither online intelligent optimization nor additional hardware, and the repeated tests show small dispersion. Cross-operating-point startup tests further identify the applicability boundary of fixed feedforward, demonstrating good engineering feasibility.