I don't think we should expect alien mission architectures are likely to look like this in terms of mass or size unless we are very near the homeworld of the al

I don't think we should expect alien mission architectures are likely to look like this in terms of mass or size unless we are very near the homeworld of the aliens and they have just started exploration. A civilization that has been building systems like this for 100s to 1000s of years would have deployed lasers along the flight path of the spacecraft allowing significantly higher and more efficient payloads. The big challenge faced by our approach is that the laser array is limited to earth orbit. As it pushes the spacecraft away from earth and the laser rapidly loses efficiency. However if you had laser arrays over the planned route of the spacecraft you can keep adding velocity. However getting that infrastructure into place decades to millennia. You start with the array in home world orbit and then start building further and further out arrays. You also want the boost stations at the destination to slow spacecraft or change their directions.