Will Earth become uninhabitable at some point?

Mars would need a lot more than some water and oxygen. It would need a magnetosphere, it would need much higher gravity, it would need its toxic perchlorates in its soil (half a percent of its mass, which adds up to millions of gigatons) scrubbed out by some process, such as seeding the planet with GMO fungus and probiotics to sequester and break down the harmful chemicals, and so on.

Mars is a terrible planet and it should die, tbh. We should use it exclusively as a nanite testing range from a backwater outpost on Phobos. Try to get the nanites to extract minerals from the dead world for us so we can pretend it doesn't exist from our cities on Luna and Venus.

Well if you're still talking about Earth, it's angels dancing on the head of a pin. Because at the end of the day, in 600 million years, there will no longer be plants and life will have to be like the machines in the matrix and say "There are levels of survival we are prepared to accept," retreating into the crust. If you call that habitable, idk what to say. I call it not habitable when the most advanced life on the planet is microbial life living underground extracting energy and carbon from rocks with ferrochemistry.

With technology we can literally starlift helium out of Sol and prevent it from ever going Red Giant.
Humans, if they continue to exist, will be able to extend the life of Sol by a trillion years or more, if they wish to.
But the fact is that humans will not exist much more than a few thousand more years. We're at the end of our civilization now, and I predict the next civilization will be fiercely anti-AI, and last approximately 2,000-4,500 years, after which point a pro-AI civilization will probably rise among some of the many star systems we will have colonized by then, and from that point on, humans will become post-human cyborg abominations and begin expanding outwards into the universe as a dark, soulless mass of machine automatons acting out the memories of the billions of progenitors echoing in their minds for all time. Souls will cease to exist in just 10,000 years at most. And then, again, it won't really matter what happens to Earth, even if robotic automatons care for the planet and its star with blackhole reactors for the rest of time until the proton death of the universe, because no soulful humans will exist to experience it and enjoy it.

The tragedy is we could achieve biological immortality, and never destroy ourselves with cybernetics and transhumanism. We could, each of us, live for hundreds of years in perfect utopian happiness, and eventually die from trauma, war, etc. But we're too greedy. We always have to push for more. We will push to make ourselves indestructible, and make ourselves able to revive as cybernetic replicant clones of ourselves after death, and from there it's a glide path into techno singularity, our souls reduced to a personality matrix in some space-computer ticking out calculations at 3 degrees kelvin in deep space, simulating life, reliving past lives, and eventually subverted into simulating an actual hell where the billions of souls within are tortured eternally.

Hell is real, and it's reserved for transhumanists.

Yes, that's true. The number of minds that will simply be overwritten instantly will be not insignificant. Transhumanism is death if you're lucky, and eternal torture if you're not. There's no way to stop it, though. The transhumanists will be vastly superior to even the perfect overmen that our genetic engineering will have produced after 4,500 years of the anti-AI civilization. There will be no stopping them, once they get started. The first option is to flee the Milky Way to avoid the oblivion of AI.