A deuterium-deuterium (D-D) startup has been proposed as a route to begin operation of magnetic-confinement deuterium-tritium (D-T) fusion power plants (FPP) without an external tritium supply. Such a pathway, if feasible, could mitigate early supply-chain constraints. However, its technical and economic attractiveness remains uncertain. In this work we present a device-agnostic, integrated analysis to evaluate the operational regime that could enable a D-D startup built upon open-source tools. The model couples plasma performance (density and temperature profiles), neutronics for D–D and D–T neutron spectra, transient fuel-cycle inventory dynamics, tritium processing times, and economic drivers. Two complementary approaches are used: (1) a target-accumulation estimate, computing the time to produce and store a startup inventory under the assumption of steady-state D-D operation, and (2) a time-dependent model of the fuel-cycle inventories to capture transient coupling between production, burn, and processing in the fuel cycle as the plasma is started in D-D and progressively transitioned up to a 50-50 D-T mixture by injecting the tritium produced on-site. Rather than claiming a single best route, our aim is to map the design space and quantify sensitivities across technical and financial levers. These open, reproducible, and extensible analyses are intended to help designers, modelers, and decision-makers assess the feasibility, timelines, and economic requirements of tritium-free or tritium-lean startup strategies.