Valley splitting is a key feature of silicon-based spin qubits. Quantum dots in Si/SixGe1-x heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 μeV in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown Si68/SixGe1-x. The valley splitting is monotonically and reproducibly tunable up to 15% by gate voltages, originating from a 6-nm lateral displacement of the quantum dot. We observe static spin relaxation times T1>1 s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, T1 is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.