A look into Simon's early life. Growing up on Mars and Eden
Simon was born on Mars in a universe where colonization of planets happened in the late 90s. That alone opens the door to a lot of questions on why humans started to seek out interstellar settlements so soon but that's a post for a different day.
Iron Lung takes place in 378 EIC (Epoch of Interplanetary Colonization) which is 2370 AD for our Earth calander and just to make it easier let's say Simon is around the same age as Mark so he's in his early to mid 30s in the movie meaning he was born around 338-342 EIC. Based on the dialog we get from Simon's mom in the flashback, they moved to Eden when Simon was old enough to at least remember his mom saying this.
Humans on average can retain verbal memories from as early as 4-5 so for the sake of the post I'll say Simon spent at least a good 4-5 years on Mars before leaving. The reason I think Simon was younger when he left Mars is because Simon associates home with Eden and if he mostly grew up there that makes sense.
In a poetic sense Simon was already born in a harsh environment. Mars is more warm then hot do to it being further away from the sun so temperatures can become freezing at night, and the atmosphere is mostly carbon dioxide so humans can't breathe the air, on top of that dust storms on the planet can last weeks to months at a time.
Now something interesting is unlike most movies and TV shows where people live on Mars in domes or tents scientists agree that realistically humans would have to live underground if settlement on Mars was possible. This is because on top of unbreathable air, dust storms and having only about 38% Earth's gravity, Mars has very high levels of radiation with very little natural protection unlike Earth, making living above ground super dangerous on all sides.
Living underground would be safer and more possible to create an artificial Earth like environment. Meaning homes, cities, medical facilities, public spaces and schools would all be underground.
So Simon would have been born in an underground society where the only domes on the surface would be used for crops, communication towers to Earth, nearby stations and solar power.
This concept art from the TV series MARS is closest to what it would look like.
Simon would identify as a Martian the same way we identify as Earthlings, it's simply the planet we're from and what memories he would have of Mars would be deeply nostalgic for him especially since he can never go back to his home planet.
Like his people he would also be raised to take great pride in being Martian. The people living on Mars are the first example of the human ability to adapt and not only survive a harsh environment on a new planet, but form a society from it mostly independent from Earth. Forming a religion around trees and foliage becomes a lot less out there when it's a human society that lives underground.
Mars and Eden's religious beliefs
While we don't know if Eden's religious beliefs began on the station or not I'd say it probably started on Mars given the last tree was brought on board. A tree isn't usually something you'd bring on a space station. So Simon was raised worshiping plant life to some extent, but it got more intense on Eden. Which makes sense, even religions on Earth all involve nature to some degree as either an element of man and life or a resource so it's logical that humans on a neighboring planet would form their own religion especially around greenery, something they literally need to stay sane while living on a desert planet underground where they can only see most foliage if they take an elevator to a surface dome.
Nature becomes less of a natural element that's meant to be left alone and more of a sacred life support that everyone is reasonable for.
This would make the people of Mars and Eden excellent gardeners, and I'd say preserving plant life is literally taught in schools. Us on Earth often take nature for granted because we have a plentiful amount of it but for a society that has a limited amount? It's protecting and nurturing life itself.
The very concept of nature existing wild and untamed with little to no human intervention would probably give Simon more anxiety more than peace.
Because he's grown up his whole life with vegetation needing to be constantly looked after, nurtured and protected in a greenhouse dome like environment. What do you mean you just leave it alone without checking on it every day? To him it literally needs aid or it dies.
For an interesting fanfiction element as well, it doesn't rain on Mars. It's atmosphere is too thin and it's temperatures too cold. It does get very thin snow but not the abundant anmount we're use too. So if Grace was able to show or simulate rain in the dome Simon who has only ever lived on Mars and a space station, would have a full-blown Peridot from Steven Universe moment.
The move to Eden was definitely something Simon was atleast unhappy about, he didn't want to go either because he already loved his home on Mars or just didn't want to move like most kids. The physical transition for Simon, his mom, and the other Martians was probably pretty seamless since they were already use to living in a enclosed environment. This is why I can see Simon actually having a much easier time in Grace's dome in Erid. Grace is used to an environment he can freely roam like the streets of San Francisco but enclosed controlled environments are Simon's normal.
It also makes a lot of sense that Eden which is a station full of Martian natives would have conflict with the C.O.I. because from their perspective Mars by the time Simon was born would be a fully functional independent society from Earth. At most they'd need occasional supplies from their mother planet but other then that there just like a sovereign nation. The community there would have existed for over 300 years. To help put that in perspective that's older than the U.S, government has existed.
Eden pre-rapture would basically be like Mars, they even wear the same desert like clothing dispite being in space meaning their clothing isn't just for protection against the elements but a literal cultural identifier. Clothing that's just meant to be fashionable would be foreign. Simon would grow up with people he already knew from Mars making it an even closer knitt community. And we know community is a big deal to the people of Mars.
Simon grew up being surrounded by people on a daily bases, a close community with shared values like planetary pride, religion, and cultural differences from Earth. Which just hits harder on how isolating prison on the C.O.I. must have felt. Simon's life would be constant human presents but then isolation for who knows how long.