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The band convergence strategy, which improves Seebeck coefficient by inducing multi-valley in bandstructures, has been widely used in thermoelectric performance (TE) enhancing. However, the phonon-assisted intervalley scattering effect is neglected and the mode-selection rules remain unclear. In this work, TE properties for $α$-, $β$- and $γ$-PbP are intestigated under the consideration of full mode-, energy- and momentum-resolved electron-phonon interactions (EPI). The group theory is used to analyze the selection rules for EPI matrix elements. Our calculations reveal that, the intervalley scattering contributes non-trivially to the total carrier relaxation time, and the intervalley scattering can be modulated through crystal symmetry. In addition, the investigation on the thermal properties reveals that four-phonon scattering effect dominates the phonon relaxation processes, since the three-phonon scattering is suppressed due to the significantly large acoustic-optical phonon bandgap in $α$-, $β$- and $γ$-PbP. By considering full EPI effect and high-order phonon scattering processes, the calculated ZT values reach 0.90, 0.24 and 1.25 for $α$-, $β$- and $γ$-PbP, repectively, indicating their promising applications in thermoelectric devices.