Improving thermoelectric performance of self-powered operation of electronics has been a continuous effort to relieve energy crisis and environmental degradation. In the branches of thermoelectric research, multiscale phonon engineering and proper doping are effective strategies to reduce lattice thermal conductivity and enhance electrical conductivity of bulk materials, respectively, yet great challenge remains in the modulation of transport properties for low-dimensional structures. Here we propose to employ doping of Ga in the single-layer octagon-square nitrogene (OS-N) as an alternative way to increase its thermoelectric efficiency. Our work demonstrates that salient electronic performance can be obtained benefiting from the convergence of valence band as well as non-localized pz orbital state. At the same time, the relatively complexed configuration of doped system shows a much increased impeding of thermal transport, which is about thirty times lower than that of elemental two-dimensional nitrogene monolayer. This pronounced improvement is attributed to the softening of phonon branches, which increase the scattering phase space and structural anharmonicity. Our work illustrates the realization of tailoring electronic and phonon properties in the dimensionality reduced system and provides a road map for potential applying it in fields such as thermal management and thermoelectrics.