The dead volume in the external pipelines of room-temperature magnetic refrigeration (RTMR) systems inevitably induces severe thermal mixing and cold energy leakage, significantly degrading the thermodynamic performance. To address this persistent challenge, this study proposes a novel flow-path optimization strategy by introducing an external regenerator (ER) to act as a physical thermal buffer. A highly compact rotary RTMR experimental platform was built, wherein the active magnetic regenerator (AMR) is packed with 0.25-0.50 mm spherical gadolinium particles, and the proposed ER is filled with 2.0 mm high-thermal-conductivity copper particles. The core innovation lies in the ER's ability to reconstruct the thermal boundary conditions at the AMR cold end by effectively absorbing temperature fluctuations during flow reversal, thereby suppressing the thermodynamic losses induced by the dead volume. A large number of experiments validate the effectiveness of this configuration. Under a hot-end temperature of 293 K, the system coupled with the external regenerator achieves a maximum no-load temperature span of 17.76 K at the optimal operating frequency of 0.2 Hz and utilization factor of 0.95, which is 34.65% higher than the 13.19 K of the original system without coupling the external regenerator. Furthermore, the ability of the system to resist external thermal loads is significantly enhanced; under optimal conditions, the maximum load-bearing capacity of this system surges from 19.4 W to 50 W, an increase of over 150%. Notably, the optimized system exhibits excellent adaptability across a broad hot-end temperature range from 293 K to 307 K, still maintaining a high temperature span of 18.87 K even when the hot-end temperature is 307 K. In conclusion, incorporating an external regenerator into the magnetic refrigeration loop provides a simple yet highly effective pathway to mitigate thermodynamic losses, offering a practical engineering solution for the development of high-performance and compact magnetic cooling devices.