Frequency-degenerate photon pairs are a key resource for quantum information processing. Yet, their generation via nonlinear parametric processes is often accompanied by parasitic interactions that hinder state purity. We present a novel approach based on dual-pump, cascaded sum-frequency generation (SFG) and spontaneous parametric down-conversion (SPDC) within a single layer-poled thin-film lithium niobate waveguide. By exploiting modal phase matching, our scheme effectively suppresses spurious photons originating from single-pump processes. This compact, single-pass architecture operates entirely in the telecom band and eliminates the need for complex filtering or microresonator structures. Experimentally, we demonstrate degenerate photon-pair generation with a brightness of (1.0 ± 0.3) × 105 Hz nm−1 mW−2 and more than 40 dB suppression of parasitic photon-pair emission. Our results establish a scalable, low-noise platform for high-purity quantum light sources and open a pathway toward on-chip squeezed light generation via cascaded processes.