Yes it is. Our ships can only go 50 light years before having to discharge. We have no idea how the Reapers surpassed this limitation, so we'd still have it if the voyage left before or during the Reaper War. And since Andromeda is a lot further than 50 light years away, going there is a problem. If that problem is suddenly solved, it is a DEM.
It would need to be a ship or ships that can make the trip without killing the crew for hundreds of years, carry the hundreds or thousands of people that are going on the mission, carry all the necessities like water and food for both the trip and the time it takes to find a place and cultivate it, a fuel tank that has enough fuel to get us there, a powerful Mass Effect drive to help us get there, tools to build once we get there, weapons and vehicles to defend in case of attack by hostiles, and so on. That project is massive, no matter how you slice it. Too large to hide before the Reapers arrive, and too large to do after they've arrived and all top minds and resources are going to the Crucible. That's not baseless assumption, that's logical deduction.
It isn't. Let's examine what we know we have in lore.
1) We have FTL. In a world where ships can travel faster than light, I feel hard-pressed to believe they couldn't build an ark travelling near-light speed. Heck, if fuel is a problem, we could use our super black hole to accelerate the ship to the desired speed. If wandering planets without engines can do that, I fail to see how a ship couldn't. We don't need fuel for deceleration, since the ark would just have to enter FTL once it reaches Andromeda. It doesn't require ground-breaking new tech. Not a DEM.
2) We have cryo-pods. We know that properly maintained, they can go up to 500 000 years. By your own count, we only need about 3500 years. Some members of the crew can be awakened regularly and any time a malfunction is detected to check it. Asari would be the ones suffering the least from being awakened from time to time - Krogan too, but I doubt Salarians would agree with being at the mercy of Krogans. If Geth and a decent AI similar to EDI are even part of the crew, that makes maintenance even easier (I'm not very hopeful about Geth being part of this venture though. We'll see). Water and food supplies can be reduced to the quantity necessary for the maintenance crew and in the end, the exploring crew. Hydroponics chambers could be set up to ensure viable food, if the exploring crew takes more time than anticipitated to find a first suitable planet (we need at least one dextro and one levo planet). As long as suitable settlements aren't found, the rest of the refugees are kept in cryo-pods.
3) We have stealth technology. The Normandy has proven numerous times it could go around undetected by Reapers, even in systems under direct attack. It would be idiotic not to incorporate such features in any ship involved in the Ark project. Besides, Reapers are not omnicient. They overlooked the Thorian, Ilos, numerous Prothean beacons, even one of their own near Mnemosyne. They don't have access to the Citadel archives anymore, and even if they did, it wouldn't be so hard to hide the Ark expenses in the Crucible ones. Mafias and corrupted politicians do that kind of things all the time. Hiding the Ark-project isn't as far-fetched as you make it sound, especially considering that at this point in the war, Reapers are concentrating their attention on the main civilisations hubs, not the remote, unhabited worlds they would only check once they crushed the main military forces.
All the necessary tech is already available, and the project can be hidden pretty easily despite it's massive scale (the Ilos facility was massive too, and yet it went undetected). It just needs really careful organisation. Is it a fool-proof plan? No. Could some of the refugees die during the travel? Of course. However, the Milky-Wayists are desperate enough to risk it.
Edit: ...Milky-Wayans? Milky-Waynese?