OPM's humans have it easy compared to the Eldian's
Maybe yes but OPM humans aren't that far behind. Don't forget that Eldia is just a part of Aot world while the whole world is in danger in OPM. Similar things happened in AoT as well but it is not as common as it is in OPM
They're protected by Heroes constantly
Hero Association didn't exist until three years ago. And even in the current point of timeline, it is not that reliable. Most HA executives are rotten with some even having monsters as pets which leads to untold casualties. Heroes are very limited in number as they are around 600 people. Even then, only 18 of them can fight actual huge threats if we exclude Saitama. Those 18 can't be considered the best of people either according to Bang. The top 4 strongest heroes can be even worse. Tatsumaki causes as much destruction as monsters, Metal Knight was about to let Z-city be destroyed, King is a fraud, and Blast almost never appears (It is kinda justified though). So much for constant protection from heroes. Speaking of Monsters, they don't have it easy either. Just look at PPP, Pig "god", Drive Knight, HA's experiments, etc. And unlike Titans, monsters are still sentient.
and based on statements in-universe, Monster's weren't even THAT common
I watched AoT over a year ago but didn't they say that titans haven't appeared for a long time in the first episode? And what they say in OPM is slightly different. Monsters weren't that common but they weren't rare either. It is just that, nowadays, their numbers are absurd. But there were a lot happening even before that. Some monsters are thousands if not millions of years old. If we look at more recent history, 800 years ago Cruel Dragon nearly made the human race go extinct. 300 years ago Ninja Leader was born and it is safe to assume that Ninja Village is as old. A prison made for most amoral people in the world was considered funny by Sonic who is a relatively good guy in the village. Then there is Abandoned Masses, a group of ninjas who made decreasing the world population their life goal, which even Ninja Village refrains contacting from. Then there are world wars that made most of the world unhabitable, completely changed climates, marked the appearance of new monsters, and possibly sank other continents.
The Eldian's have to suffer threat of being eaten alive every moment
It is not the best way to go but it is not the worst either. In OPM that and worse can happen to both monsters and humans. According to Choze, becoming a monster removes moral restrictions. In other words, they can do any kind of evil without blinking as long as it is not inconvenient to them and they do. There are also monsters like Royal Ripper who is feared even among other mysterious beings and he roamed around long enough to be in books. There is no shortage of these monsters since you can transform into a monster just through a surge of negative emotions which is also one of the worst ways to go. Someone will inevitably suffer.
forced to live like cattle waiting to be slaughtered and are trapped within a small region
OPM puts laughter into slaughter and it is easily comparable to Aot in that aspect. The aforementioned Ninja Village, Abandoned Masses, Cruel Dragon, World Wars alone are enough. Even other races aren't safe. Boros who destroyed A-city as soon as he appeared and destroyed countless civilizations previously or "God" who makes Boros look like a joke. Blastice league and previous interstellar destruction are proofs of that.
Being trapped in a small area is very relative. OPM humans can't move around freely either. They are surrounded by countless dangers. Underground? Seas? Sky? Neighborhood? Other continents? Outer Space? Other Dimensions? All of them contain someone against humans one way or another. Even nature itself tries to kill humanity. Just one continent which is not fully inhabited is way too small against those things.
and even then, humans still fight amongst themselves
That happens even in real life. Even if you said that it is particularly bad in AoT, I can say the same about OPM. I don't even need to mention previous things. MA is led by a human, Eleven and Otento are a bunch of humans, Organization is made by humans, House of Evolution is founded by Genus, Thief Guild was mentioned by Tongara among other things, a good chunk of monsters are former humans like titans, countless organizations were destroyed by Saitama, Genos, Flashy Flash, etc. Just look at people killed by Flashy Flash. All of them are dead despite being mentioned randomly. It goes to show just how many of these guys met their doom thanks to Flash. And there is even more out there, each having their own underground army.
look at what was going to happen to Mikasa had Eren not come by.
Look at Psykos' experiments.
I probably forgot most of what I was gonna say while writing this. This is just a type of the iceberg
