Colonizing the bottom of the ocean would be orders of magnitude easier. Or the South Pole. Or Kīlauea’s open lava pit.

While it is true that you have different sets of problems to solve, nope, they are only cheaper to get to, not necessary easier to colonize, except maybe the South Pole where you just need to build something that only need to withstand the cold, which is easy enough and you could go outside without a space suite or something similar.

The problem with colonizing Mars is the cost, which have as a consequence the cost of everything you send to that place.

But in the end I think that we already have all the basic blocks for a base on Mars (or the Moon) and what it is stopping us is the cost of putting everything together and send it.
We already know how to build isolated environments that can must stay sealed for month or years (subs and International Space Station), we already know how to recycle things like air and water, we already know how to produce vegetables in cramped spaces and with low or no exposure to the sun (think of every weed farm inside houses ;-) ) and minimal water needs, we already know how to develop and deploy complex industrial control systems and so on.