Where comes the assumption that Reapers' wouldn't harvest other galaxies?
Because while I admit that anything is possible here, the framing in the trilogy was that:
1)The Leviathans only ruled the Milky Way, not beyond it, and did not express interest in beyond the Milky Way
2)The Reapers/Intelligence were using the Milky Way as an experiment, so even if the experiment's successful results could apply beyond the Milky Way, there is no evidence that the Reapers have multiple experiments happening at once, but instead they focus on the Reaping cycle within this one galaxy
3)The Reapers conserved energy by sleeping in dark space between galaxies, to be awoken by their vanguard, Sovereign
4)While godlike in power for the Milky Way, the Reapers may in fact be aware that they could be small fries in the supercluster/universal/multiuniversal(?) scale*, so they act relatively safely and efficiently and keep their experiment restricted to the Milky Way until results are found (given that time matters little to them)
Any new information could come up, and things are left vague enough on all 3 of those points for that to happen, but still, the framing of the trilogy story does not give us any information about anything happening beyond the Milky Way. At best, we get little hints like from TIM "Against the Reapers.. and beyond", but that doesn't really indicate that the Reapers have/have had a presence in other galaxies.
Personally, I consider the Reapers like the DA Templars in a few regards - important for the sense of order over the Milky Way/Southern Thedas, but even if they had influence over other galaxies/Northern Thedas, it was limited at best (Tevinter Templars, Anderfels' remoteness and focus on Wardens instead, Qunari land, Rivain's apathy for Templars, etc).
I'd less believe that the Reapers harvest other galaxies as I would wonder that individual Reapers had the agency to sometimes explore other galaxies for information and technology, not reaping of organics.
In fact, I'd kind of like that. 100% haters of Reapers (as in people who resent and reject any lore information that doesn't show the Reapers as either complete monsters or completely malfunctioning robots) would chafe at it, but it would be an interesting message to see that the Reapers as a faction/its leading figures never held any actual malevolence for organics and could even deal with them outside the Milky Way if deemed necessary (like Cerberus dealing with the Council/aliens regardless of their human-supremacy intent). Never means we should ever side with them, but it'd be a nice tie to the trilogy and give us Reaper lore (which can be made relevant to some secondary and primary plotlines) without the next game being a 'Reaper Story'.
EDIT: *The awe indoctrinated organics may feel for the Reapers is still legitimate in the story. It may be understood as one of the only ways that a lesser organic mind may comprehend anything of the Reapers' while still connected to them neurally, and it may be understood as a useful tool for the Reapers to keep organics pacified. I don't think anything in the trilogy story actually declares the Reapers as gods, but the line in ME2 that explains 'god as a verb' is potentially significant. My point is that the Reapers, despite their quasi-religious aspect, definitely don't need to be the be-all-end-all of 'power' in the Mass Effect universe, and there can be powers well beyond even them. At the same time, I wouldn't personally mind if story aspects of the Reapers stuck around (just not emphasized anymore), and I'm open to several routes Bioware could take with that.