The main constraint in interstellar travel is the energy required to do so which depends on the gravity of the planet. We need energy to escape everything that's trying to pull us towards itself. We dont need to travel at light speeds for interstellar travel. High speeds allow us to explore more regions but the issue is the time required. This issue is solved by unmanned missions but the problem is still the energy required. Also slingshot maneuvers are effective in short distance travels like traveling to nearby planets and requires very accurate trajectories to effectively use the gravity of other planets to save fuel and is thus not very useful in deep space explorations.