[Objective] Tethered heavy-payload firefighting UAVs performing sustained hovering at 200 m altitude face critical challenges: Insufficient power density, excessive winding temperature rise, and inadequate 100-hour operational reliability. Addressing performance limitations in conventional external-rotor permanent magnet synchronous motor-specifically high AC copper losses from round-wire windings and low air-gap flux density due to parallel magnetization, this study proposes an integrated solution combining multiphysics collaborative optimization with innovative topological design. The approach targets stringent triple requirements for electric propulsion systems: lightweight construction, minimized heat dissipation, and superior operational robustness in aerial firefighting scenarios. [Methods] Using a multi-parameter sensitivity hierarchical degradation model validated for predictive accuracy, parameter co-optimization for an external-rotor motor with 36 slot/32 pole was performed via response surface methodology. The design incorporated a five-segment Halbach magnet array topology to enhance fundamental air-gap flux density while suppressing torque ripple through optimized magnetization angle distribution. This was combined with flat-wire winding technology and a dovetail-groove stator design, achieving a 78% slot fill factor and reducing AC copper losses via three-dimensional end-turn optimization. Pareto-optimal solutions balancing thermal stability and efficiency were selected using a genetic algorithm based on a composite objective function. Prototypes underwent rigorous validation through propeller dynamometer tests simulating full-load aerodynamic profiles and flight demonstrations at 200 m altitude under variable atmospheric conditions. [Results] The optimized motor achieved a power density of 3.73 kW/kg with 92.9% system efficiency under rated propeller load. Winding temperature rise stabilized at 118 K, the electric drive system operated stably during the 200 m altitude flight test. Following 100 hours of continuous flight, efficiency degradation remained below 0.1%, outperforming similar products. [Conclusion] The hierarchical optimization framework synergized with Halbach-flat wire winding technology, achieving unprecedented power density and operational reliability essential for prolonged firefighting missions, while offering a scalable methodology for high-altitude electric propulsion design.