Looks good to me as a rough back-of-the-envelope calculation to get the orders of magnitude right (the speed will go down as the object is climbing out of Sun's

Looks good to me as a rough back-of-the-envelope calculation to get the orders of magnitude right (the speed will go down as the object is climbing out of Sun's gravity well). However, you're still missing a guesstimate for alien lifespan. Do you have reasons to believe they live shorter than 33552 Earth years? Also, is 38 km/s really the top speed for hypothetical interstellar ship? I imagine aliens that live on average 870 Earth years (~10x average lifespan of a female Japanese) and travel at 9,000 km/s. They can easily make Sun-Proxima Centauri trip six times in a lifetime.