If we are receiving the FRBs, the focused beam must be pointed towards earth. So the spaceships with light sails must be headed in the direction of earth. Get ready for the invasion!
1. the wavelength is relatively long, because you want a sparse microwave sail
2. the antenna is of finite size
3. the distance involved is intergalactic
Come to think of it, even if they were aiming directly at where we are now I would assume the light source would still be occulted regularly by everything between us and them.
A lot of people talk about bussard collector (magnetic scope for hydrogen/helium) to power fusion. But instead of collecting gas use it as a drag chute.
It could be an intra(inter)-galactic ballistic missile (i.e. no deceleration);