The complexities of hardware and signal processing make it especially challenging to develop self-interference cancellation (SIC) techniques for full-duplex (FD) massive multiple-input-multiple-output (MIMO) systems. This paper examines an FD massive MIMO system featuring multi-stream transmission. Specifically, it adopts an architecture where a single transmit or receive radio frequency (RF) channel is connected to three antennas in the same polarization direction, effectively reducing the number of transmit and receive RF channels by half. The SoftNull algorithm serves as the primary method for SI suppression, leveraging digital precoding during transmission. Additionally, this study outlines a design strategy to enhance SIC in the proposed system. Simulation results highlight the efficacy of the SoftNull algorithm, which achieves a remarkable total SIC of up to 64 dB. Furthermore, combined with measures such as antenna isolation and increased transceiver array spacing, the resulting sum rate can be twice that of a half-duplex system.