This study addresses the critical challenge of low reactivity and environmental leaching associated with high-volume phosphogypsum by implementing an ettringite seed induction strategy to optimize foam concrete performance. These issues may lead to insufficient mechanical reliability, weak structural integrity, and environmental safety concerns in engineering applications. The results demonstrate that an optimal 2% seed dosage increases the 28-day compressive strength to 4.7 MPa, which represents a 150% improvement over the control while maintaining mass loss below 3.0% after 15 wet–dry cycles. Microstructural analysis reveals that the seeds serve as heterogeneous nucleation sites that help mitigate the inhibitory effects of phosphogypsum impurities to facilitate the growth of a dense 3D interlocking ettringite framework within the pore walls. This densification significantly reinforces the mechanical skeleton and reduces phosphorus leaching by 64.2%. However, excessive seed dosages at or above 5% may promote local AFt accumulation and rheological slurry-bubble mismatch which could contribute to microstructural defects and strength regression. The findings are expected to provide a scientific basis for the engineering application of high-volume phosphogypsum in foam concrete, particularly as lightweight fillers and non-load-bearing construction materials for sustainable construction.