This does create slight plot hole tho, right? I mean, the arks can't possibly have non-reaper tech base, whole galaxy technology is reaper based as stated in ME trilogy. The initiative didn't have enough time OR resources to develop a new drive core from nothing (were too poor to even do anything until the mysterious benefactor showed up). It had to base on tech left by reapers for other races to ascend lore-wise or it would take them tens of years to develop new and unique drive. It could be built upon reaper tech as in upgrading it but that still makes a plot hole, because wouldn't the reapers have issue with the races they came to harvest to escape like that to another galaxy that is outside their reach? Again, this makes no sense.
The citadel relay purpose isn't only needed for reapers to get back in time, it took them not that long between ME1 and ME3 timeline and again, we don't know how deep in dark space the reapers were waiting. Most of citadel's purpose is to gather the galactic rulers in one place (council, seat of prothean empire) and get rid of the leadership thus crushing possible resistance. It's second purpose is to also disrupt communications. As Javik pointed out, the citadel was first to fall, which left the worlds with no warning and no mobilization for reapers to pick them one by one until it was too late. So all in all, it MIGHT be possible for Reapers to actually go to other galaxies.
As for the absence of mass relays, they might not have been built by the reapers though but by Leviathans, who are shown to have traveled Milky Way. We don't know if Reapers build anything actually, what we know is that Keepers were doing their work of keeping the citadel ready for next cycle and we can assume they did likewise for the relays.
As for the info how many reapers are created each cycle we know that from leviathan dlc where you literally are told "Each harvest ends with the birth of the new reaper". This quote comes from the game itself so it's a confirmed info there is one, as in one reaper stores all knowledge of one cycle.
The remnants though is a good point for you to bring up as well as the civil war. It would point to the conflict of synthetics wiping out organic life. I am unsure where I stand with Bioware potentially bringing in reaper lore to Andromeda, it could be done right but as you said, it's an old stuff and that would be reusing it. However I would welcome for them to not completely ignore this, just don't make it main plot or something.
Thanks for this conversation. It helped me sort out some thoughts as to why the cycle of harvest doesn't appear in Andromeda. The reapers are machines which operate in their own set way. If Leviathans didn't build mass relays to go to andromeda, it means the reaper activity is indeed limited to the galaxy of their creators. I think the reapers just leave Andromeda be while it follows the route of apex race being wiped out by synthetic life. While reapers clearly COULD go there, they simply don't, it's not their test zone to find the solution they were tasked with.