Deep oil and gas development is critical for ensuring stable fossil energy supply and facilitating a smooth energy transition. However, under extreme high-temperature, high-salinity, and high-density conditions, the weak network framework of water-based drilling fluids often causes uncontrolled rheology, fluid loss, and poor sedimentation stability. Conventional rheological modifiers strengthen the structure but induce excessive viscosity, creating trade-offs among key performance properties. In this study, a novel zwitterionic polymer modifier (LRM) with an aggregated cluster structure was synthesized, featuring low viscosity and high yield point behavior. LRM significantly improves yield point, reduces sedimentation, and enhances mud cake compactness at 200 °C, 30% NaCl, and 2.4 g/cm3 density, without notably increasing viscosity. Its aggregated clusters act as anchoring points, reinforcing the framework. In high-salinity environments, the anti-polyelectrolyte effect extends LRM chains, promoting multipoint adsorption and bridging with clay particles, while curled chains maintain low viscosity. The unique spatial structure buffers stress and forms recoverable micro-clusters with strong viscoelasticity, giving the fluid high initial resistance to flow and a low-viscosity, high-yield point profile. Furthermore, LRM also shows excellent compatibility with sulfonated, polymer-based, and clay-free systems, enhancing performance under harsh conditions. These results demonstrate LRM's potential for designing high-performance drilling fluids with controllable rheology, low fluid loss, and improved stability.