Will never happen without major science fiction grade technology, the atmosphere is way too thick to even think about sending anything larger than a tiny probe into.
What do you mean?
Venus has already received its fair share of probes, the Venera missions, some weighted upwards of five tons
https://en.wikipedia.org/wiki/Venera
They did many different types of measurements, and lasted from one to two hours before their batteries running out, some atmospheric probes lasted days
Sometimes the will just isn't there.
(Gavin Menzies, the author behind the claim China circumnavigated the world before Columbus was born, wrote a third book about how Atlantis did it first. It was equally convincing.)
And I don't see how a colony on Mars even helps with probes. We send material from Earth to Mars and then send probes from Mars? How is that more efficient? Do we even know what materials could theoretically be mined on Mars? And probes are launched with gravitational assists.
Humans are evolved to fit this gravity well. We use the gasses and metabolites here. Trying to leave the planet and make it all work in the dark, cold, bleakness of our solar system is ridiculous. Our lives are fragile and our lifespans too short to make anything offworld viable.
Our robot descendants will inherit the stars instead. There's nothing wrong with human life being just a stepping stone to bootstrap the hand off.
“Trying to leave the savanna and make it all work in the dark, cold, bleakness of Europe is ridiculous.” - some early hominid, fairly recently
We should empower them to go where we can't. In a sense they'll be our children, carrying the torch and memories of humanity with them.
Like if they have a society, maybe their founding myths will involve a baby robot created by some (non-human) robot deity, or if they are more rational a pseudo-scientific story of evolution from a toaster.
And once we had done that, we couldn't go back in significant numbers before inventing sun cream to functionally replace the melanin our bodies no longer produced in enough quantities to prevent our skin burning and developing cancer just from being in the sun.
Technological progress may make this all faster than evolution, but a mechanism that's optimised for Venus may freeze solid on the hottest day in Death Valley, Earth.
Ask yourself why. What's the real job to be done by humans on Mars? What's the real economic situation? How does it map to human desires?
Robots will do the job even better than us and will see 10,000x the investment. Evolution follows gradients of least resistance.
Human history offers plenty of precedent for this; "we want to run things our own way", "all the land is taken where we live", etc. I don't wanna live in Scottsdale, AZ either, but someone does.
(And I'm saying that as an optimist because I want the billionaires to develop that tech: the ability to fully terraform Mars into a self-sustained world is necessarily also sufficient to handle any imaginable damage to Earth up to and including a surprise impact with a moon-sized rock that excavates the crust and mantle everywhere to a depth of a few thousand km, meaning that things like greenhouse gases and biodiversity would become as easy to fix in that future as a single broken window is today; even just the bare minimum of a single self-sustained domed city on Mars means most environmental issues have to be "solved").
Nevertheless, I think "my skin burned and blistered" is a bit more immediate with the painful feedback, even if it's not as extreme.
Nobody is going to want to live through either those days or those nights
The only thing venus has, that makes it interesting for human settlements, is earthlike gravity - but the rest is pretty hell like.
The Earth weighs more than everything else between the sun and Jupiter combined, and Venus is 80% of that.
But yes, I also see no way of "fixing" the g of mars(which is likely way too low for humans), unless we go into the buisness of planet building.
We don't really have a way to know until we get experimental data (real humans on Mars). Until then, it's all just (un)educated guesses.
We know that long time weightlessness causes damage, so 38% of earths gravity will likely have some bad effect, too. This is not just wildly speculating. But yes, for finding out, whether humans still can bear it, we will have to find out and there is no shortage of volunteers.
First you want to get your self-replicating probes occupying the Oort cloud, whatever. Get ready to sort and aim. You start with your "rocky" bodies, skipping them off of Venus' atmosphere, just ripping it off in big hunks, until you're down to about one Earth atmosphere.
Next, you flat up drop your icy bodies into Venus, because it really needs water.
During that time you'll want to start constructing your insolation shield, to help cool things down.
Only after that can you move Mercury into place.
The outgassing CO2 could be captured and turned into carbon and oxygen, with the carbon put to industrial use or into biomass and the oxygen feeding a 400 millibar O2 atmosphere in settlements.
But a sudden volcanic triggered burst of CO2 would be a forever ongoing global risk.
The Soviet Union did send probes to Venus back in the 70s. But, indeed, it would be great to have new and higher resolution photos. I am always amazed when looking on the surface of a foreign world.
EDIT: oh and there is oxygen everywhere, if your spacecraft doesn't corrode it will burn. And even if it lands 2/3 of this planet are covered with 4km deep oceans of the corrosive stuff!