Many basic materials, like water, make great shields if necessary. For Mars one plan is simply to organize the ship such that the water reserves can act as an radiation shield when necessary. You don't need anything particularly massive or overly fancy.
The problem is that paradoxically, thicker shields can cause higher doses of radiation due to secondary radiation production. Theres a balance there to minimizing primary and secondary dosage but you cant really get it below a certain threshold without some form of active shielding. Theres a lot of promising ideas for active shielding but they all require alot of power. All this to say that someone on a trip to mars will indeed get a high dose and this is something that needs to be given thought. Its not an intractable problem but it is also not even close to being solved or trivial.
Folks on Mars is still quite a ways away. Not impossible or pointless by any means but it will require significant advancements in many domains before we can really do it safely.
The secondary radiation you're talking about is called bremsstrahlung (braking) radiation [1]. This is precisely why water is, to my knowledge, the current leading idea. It works not only as an excellent primary shield but also helps to minimize secondary radiation, and you also naturally have literally tons of it.
As an aside, this is why the original Moon missions used simple aluminum. One of the typical angles people who don't think we landed on the Moon used is that to safely get through the Van Allen Belts we would have needed massive lead shielding on the craft, when in reality it was shielded by a pretty thin layer of aluminum, precisely to minimize secondary radiation. If you put that thick lead shielding on the craft that people intuitively think you'd need, it very well could have killed the astronauts.
Polyethylene is similarly a good shield because its got high hydrogen content so is pretty good at moderating neutrons. I image a polyethylene structure containing a layer of water is probably something like what we'll end up settling on. Its also quite strong and would make decent structural components.
1m3 of water weights a metric ton, do the astronauts stay in a broom closet for duration of the flight? You still need (literally) tons of water.
And every ton you take increases fuel cost.
Currently you need what 10-20 trips to fuel for moon. How many trips would you need to fuel trip to mars? You would need a fucking conveyor belt of starship to prep the mission. Who is going to pay for it? And why?
Just because the materials for shielding/tools are simple it doesnt make the solution simple.
You hit on two really different topics at the same time. So let's hit on the fuel topic first. People tend to draw intuitive parallels of rocket fuel with gas. A short trip doesn't need much, while a long trip needs a lot. But it turns out that's not really how it works in space. In space the way you travel is you point at something, accelerate a bunch, and then coast there. So the typical measurement for something analogous to fuel is called delta-v, or the total required change in velocity. The delta-v required to get to the Moon is around 19km/s. The total delta-v required to get to Mars is around 21km/s. Wiki has a nice table for the delta-v required to get to different locations here. [1] Not only are they very close, but you might also notice that you spend about half your delta-v budget just getting off of Earth's surface! That's why in-orbit refueling is so useful.
On the water topic, here's [2] a very readable little writeup on the mass inputs required for longterm space missions. Water makes up the overwhelming majority of your requirements. Starship, in its current form, can carry 150 tons. And that will be substantially increased once the tech is stabilized. A sizable chunk of that storage, for Mars missions, will be water. Also, the big threat is not the persistent ambient radiation, but infrequent extremely high energy bursts from solar weather.
Why does this matter? The delta V requirements are so steep Starship can't return from the martian surface. It is a completely different game to go to Mars than to the moon unless you are planning on building a mars gateway in orbit and never landing anywhere.
Oh this opens a whole bunch of really interesting topics. One of the most interesting is ISRU - in situ resource utilization. Using what you have available to make further progress. The Sabatier Reaction [1] is one of the single most important factors in our future expansion. Basically, CO2 + Hydrogen => Methane + Water.
Mars has essentially infinite CO2 and hydrogen. So this means we can produce essentially infinite methane and water. In fact you can carry out electrolysis on some of the produced hydrogen and feed it back into the system. This leaves you with a final excess product of methane, oxygen, and water. The Starship's engines, uncoincidentally, run on oxygen + methane. So we can locally produce basically limitless amounts of water, oxygen, and rocket fuel on Mars.
Of course this begs the question of what we do initially. And like all long distance colonization efforts, you're not sending just a single ship, but rather a small group. Those other ships are carrying additional critical supplies, including fuel. Upon landing those ships can also be used as shelters until something more permanent can be constructed. And the extra fuel provides a means of getting back to Earth if necessary while giving you a big window of time to start establishing basic local infrastructure including energy and methane production.
more interestingly, the impact on human psyche is basically unknown. We have meh approximations but just like placebo you cannot fake some things, like 'you are not coming back' condition in an experiment.
The permanent dwellings and really whole infrastructure would be underground. Lack of atmo and magnetic field basically forces you to dig down. Limited resources - rudimentary food and water. More less bunker life
On top of that 0% prospect of return. Essentially you are doomed to penal colony for the rest of your life.
I find it hard to believe anyone would want to sign up to that. And the people who would are absolutely people who should not be sent there.
And what happens if earth decides supporting mars outpost is pointless, you want alex jones types create conspiracies that colony is fake and its a tax payer scam? What would that do to minds of those stuck on mars?
You're making a lot of assumptions without looking into anything. So for instance the ambient radiation on Mars is comparable to Ramsar, Iran. [1] It's the most naturally radioactive colonized location on Earth, yet people have been living there for generations with no concerning side effects whatsoever, and a number of seemingly positive effects. The bigger threat is actually quite similar to the issue with space - it's the intermittent major space weather events sending short extremely high intension bursts of radiation at you. Fortunately these can be detected and dealt with as necessary.
And the Mars program is not being publicly funded in any way, shape, or fashion. If it was - we would have likely colonized Mars decades ago. Werner von Braun - the man responsible for the success of the Apollo program already had workably viable plans drawn up for Mars before we even set foot on the Moon. The government's decision to defacto cancel the space program is what led to his very premature retirement. But I do think rapidly aiming for the basics of self sustainability will be important nonetheless. Fortunately Mars is almost set up like a video game in that most of everything we need to survive is somewhat serendipitously present.
---
As for what people want to do. It's funny, because I feel the exact opposite. I simply cannot understand people who have no desire to adventurize, explore, and see all that this universe has to offer in what little time we have on this Earth. It's probably why I'm an American yet find myself living half way around the world. And I would be the first to sign up for a mission to Mars as well. It'd certainly be a rather difficult life, but what more could be more desirable than spending the rest of your years laying the foundation for our children and our childrens' children, so that one day people coming to Mars would simply be just a holiday. And perhaps they then will then be the ones laying the foundation on Europa, or perhaps destinations that we can't even realistically imagine today.
It's certainly the same mindset people leaving the luxuries of the Old World had when setting out for the New World. The Old World was richly developed and full of culture, life, and civilization. Going to the New World entailed a voyage, as long as 4 months, and one which was quite frequently deadly. All to get to a completely undeveloped chunk of land full of oft unfriendly natives, new diseases, and all sorts of great ways to die in the middle of nowhere. And that's if you survived the voyage across to begin with, which many didn't. In many ways, we have it easier.
