The Economic Viability of Mars Colonization (1995) [pdf] (opens in new tab)

To survive long-term without getting massively irradiated you need, at a minimum, metres of rock between you and the sun (and as you say, lifting them from Earth is going to be expensive). Ideally a magnetosphere too. Gravity is also desirable for health reasons and random practicality things (e.g. in zero-g you have to sleep in a draught, as otherwise there's a risk of a dangerous bubble of CO2 forming around your head). And temperature can be a huge problem for spacecraft that aren't attached to a big heat reservoir - scorching in the sun, freezing in the shade.

Maybe ultimately we'll be able to construct nice habitats for orbiting stations, but with current engineering capabilities rocky bodies are the only safe places to stay long-term. I certainly expect we'll get to asteroids eventually. But for now, Mars is closer than the asteroids, closer in delta-V terms than the Moon, and much more promising in terms of having organic-y raw materials available for farming and fuel. It makes sense to go there (or maybe Venus, but aerostats would be a big chunk of new engineering) first.

It isn't an either or.

Colonizing planets and producing space stations would only help enhance the technology, provide refueling and repair places, and much, much more. People will have preferences on where they want to live (dirtside or not) in the same way people have preferences on where they live now. I think it would be safe to assume there would be a little of both (spaceside and dirtside) living out there. (I think that resource-wise, there will be more on planets because of its benefits)

Also worth mentioning is the Mars Underground documentary, detailing Zubrin's work and mission over the years:

https://www.youtube.com/watch?v=tcTZvNLL0-w

It's also kind of vital for the balance of the planet's ecosystems, as we're slowly finding out. If we melt it to get materials under there, there's a sizable chance of long term regret.

Whereas Mars doesn't have an ecosystem at present and no species to endanger.

I want to make other planets livable so we can stop killing this one so much.

Afaik, it's viable but there is a global ban on Antarctica exploitation till 2048.

Forgive my ignorance but what are the implications of no magnetic field on humans, mammals etc? Does it regulate some crucial biological function(s)?

Yes, that's a big issue for living on the surface. On the other hand the atmosphere is a nice trivially accessible source of a number of important chemicals and makes getting from space to the surface much easier.

Why not use an orbital mirror that reflects only the wavelengths desired?

Except that despite beautiful renderings of cities on the surface of Mars, the radiation would force everyone to live underground. And if we are going to build huge cities under the surface of Mars, we might as well try it here. But no one is doing it,because there is no point.