Dual-fuel combustion is emerging as a promising solution to address the growing focus on maritime decarbonization, because it is adaptable and needs minimal system modifications. However, natural gas as an alternative fuel must deal with the issue of methane slip, because methane has greater global warming potential than CO2. Conventional aftertreatment systems may incorporate a methane oxidation catalyst to mitigate methane emissions, but effective methane oxidation requires high temperatures of approximately 400 °C. Therefore, exhaust thermal management (ETM) is crucial for maintaining high exhaust gas temperature (EGT) and ensuring conversion efficiency. This study investigates the effectiveness of fully variable valve actuation (VVA), including early exhaust valve opening (EEVO) and early intake valve closing (EIVC), along with lambda control via wastegate control. Each strategy’s effect on exhaust gas temperature is evaluated, while considering potential trade-offs with efficiency. The research uses a model-based approach, simulating a state-of-the-art, six-cylinder natural gas/diesel dual-fuel marine engine (Wärtsilä 6L20 DF), equipped with a two-stage turbocharger with wastegates. Numerical simulations are conducted using a one-dimensional (1D) engine model within GT-Suite across two different load conditions. The model is validated using baseline valve timings and a comprehensive dataset of experimental data. Results indicate that all three strategies can contribute to EGT elevation. EEVO raises EGT by 73 K, but incurs a 3.85% reduction in brake thermal efficiency (BTE). EIVC achieves a substantial EGT increase of 122.7 K at medium load, with a slight BTE improvement of 0.4%. Wastegate lambda control elevates EGT by 91.5 K at low load, exhibiting a negligible BTE impact. Thus, VVA-based ETM and lambda control enable rapid warm-up of exhaust aftertreatment systems (EATS) in large-bore engines with a minor efficiency penalty. This helps compliance with stricter emission regulations which contribute to maritime decarbonization, eventually enhancing air quality and the maritime ecosystem.