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 mu eV in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown Si-28/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 T-1 > 1 s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, T-1 is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.