It's amusing how there are all these incredibly intimidating and god-tier things believed about the billion-year-old Reapers, yet everyone assumes they can't just travel to Andromeda like our rinky-dink tech apparently can. Why is it safe, and why would we assume it's beyond their grasp?
You're misunderstanding the difference between intention and capability again, so I'll make it really clear:
The Reapers probably CAN travel to Andromeda. That doesn't mean that they DO.
We have no reason to suspect that they've ever been there. None of the Reapers we've talked to, or the Catalyst, have talked about a mission that extends beyond our own galaxy. Indeed, the Catalyst presents the Crucible and Synthesis - which only effect the MW - as his perfect final solution to the original problems the Reapers were created to solve.
But let's assume for a second that you're right and the Reapers do regularly harvest Andromeda, too.
If their mission is generalised (i.e. it applies to harvesting all life, everywhere) this introduces a number of other problems to the lore of the series - like exactly how far the Reapers regularly travel, how many galaxies they visit, the logistics of harvesting multiple galaxies simultaneously (because eventually it would take more than 50,000 years to reach the furthest ones) and the problem of the Reapers' numbers.
If they harvest Andromeda, why stop at Andromeda? Where do they stop? If they have harvested dozens or hundreds of galaxies, how do they harvest them all in the space of one cycle? If it takes multiple cycles, do they have multiple fleets? Why is the Reaper fleet we face in the Milky Way so small if they have harvested civilisations for a billion years from multiple galaxies?
If you're correct and their mission extends to harvesting all life, the 50,000 years spent in dark space between cycles is an utter waste of time - so, is there another Reaper fleet in Andromeda? In every galaxy? Will they come back to the Milky Way once they notice we've beaten the Reapers?